Help: Better explain CMAKE_<LANG>_EXTENSIONS_DEFAULT

Explain that this represents the compiler's default and mustn't be modified
by the user. Clarify when it's used as the default.

Additionally:
* Add a reference to it in cmake-compile-features in text explaining the
  feature.
* Add explanations for the default initialization by
  `CMAKE_<LANG>_EXTENSIONS_DEFAULT` to all `<LANG>_EXTENSIONS` pages and
  references to CMP0128.
* Slightly reduce the wordiness of the default initialization explanations by
  removing an unnecessary "it is".

Fixes #22828.

Help/manual/cmake-compile-features.7.rst

@@ -119,9 +119,9 @@ Availability of Compiler Extensions
 -----------------------------------
 
 The :prop_tgt:`<LANG>_EXTENSIONS` target property defaults to the compiler's
-efault. Note that because most compilers enable extensions by default, this
-may expose cross-platform bugs in user code or in the headers of third-party
-dependencies.
+default (see :variable:`CMAKE_<LANG>_EXTENSIONS_DEFAULT`). Note that because
+most compilers enable extensions by default, this may expose portability bugs
+in user code or in the headers of third-party dependencies.
 
 :prop_tgt:`<LANG>_EXTENSIONS` used to default to ``ON``. See :policy:`CMP0128`.
 

Help/prop_tgt/CUDA_EXTENSIONS.rst

@@ -15,5 +15,7 @@ See the :manual:`cmake-compile-features(7)` manual for information on
 compile features and a list of supported compilers.
 
 This property is initialized by the value of
-the :variable:`CMAKE_CUDA_EXTENSIONS` variable if it is set when a target
-is created.
+the :variable:`CMAKE_CUDA_EXTENSIONS` variable if set when a target is created
+and otherwise by the value of
+:variable:`CMAKE_CUDA_EXTENSIONS_DEFAULT <CMAKE_<LANG>_EXTENSIONS_DEFAULT>`
+(see :policy:`CMP0128`).

Help/prop_tgt/CXX_EXTENSIONS.rst

@@ -15,5 +15,7 @@ See the :manual:`cmake-compile-features(7)` manual for information on
 compile features and a list of supported compilers.
 
 This property is initialized by the value of
-the :variable:`CMAKE_CXX_EXTENSIONS` variable if it is set when a target
-is created.
+the :variable:`CMAKE_CXX_EXTENSIONS` variable if set when a target is created
+and otherwise by the value of
+:variable:`CMAKE_CXX_EXTENSIONS_DEFAULT <CMAKE_<LANG>_EXTENSIONS_DEFAULT>` (see
+:policy:`CMP0128`).

Help/prop_tgt/C_EXTENSIONS.rst

@@ -15,5 +15,7 @@ See the :manual:`cmake-compile-features(7)` manual for information on
 compile features and a list of supported compilers.
 
 This property is initialized by the value of
-the :variable:`CMAKE_C_EXTENSIONS` variable if it is set when a target
-is created.
+the :variable:`CMAKE_C_EXTENSIONS` variable if set when a target is created and
+otherwise by the value of
+:variable:`CMAKE_C_EXTENSIONS_DEFAULT <CMAKE_<LANG>_EXTENSIONS_DEFAULT>` (see
+:policy:`CMP0128`).

Help/prop_tgt/HIP_EXTENSIONS.rst

@@ -15,5 +15,7 @@ See the :manual:`cmake-compile-features(7)` manual for information on
 compile features and a list of supported compilers.
 
 This property is initialized by the value of
-the :variable:`CMAKE_HIP_EXTENSIONS` variable if it is set when a target
-is created.
+the :variable:`CMAKE_HIP_EXTENSIONS` variable if set when a target is created
+and otherwise by the value of
+:variable:`CMAKE_HIP_EXTENSIONS_DEFAULT <CMAKE_<LANG>_EXTENSIONS_DEFAULT>` (see
+:policy:`CMP0128`).

Help/prop_tgt/LANG_EXTENSIONS.rst

@@ -14,7 +14,8 @@ These properties specify whether compiler-specific extensions are requested.
 
 These properties are initialized by the value of the
 :variable:`CMAKE_<LANG>_EXTENSIONS` variable if it is set when a target is
-created.
+created and otherwise by the value of
+:variable:`CMAKE_<LANG>_EXTENSIONS_DEFAULT` (see :policy:`CMP0128`).
 
 For supported CMake versions see the respective pages.
 To control language standard versions see :prop_tgt:`<LANG>_STANDARD`.

Help/prop_tgt/OBJCXX_EXTENSIONS.rst

@@ -18,5 +18,7 @@ If the property is not set, and the project has set the :prop_tgt:`CXX_EXTENSION
 the value of :prop_tgt:`CXX_EXTENSIONS` is set for :prop_tgt:`OBJCXX_EXTENSIONS`.
 
 This property is initialized by the value of
-the :variable:`CMAKE_OBJCXX_EXTENSIONS` variable if it is set when a target
-is created.
+the :variable:`CMAKE_OBJCXX_EXTENSIONS` variable if set when a target is
+created and otherwise by the value of
+:variable:`CMAKE_OBJCXX_EXTENSIONS_DEFAULT <CMAKE_<LANG>_EXTENSIONS_DEFAULT>`
+(see :policy:`CMP0128`).

Help/prop_tgt/OBJC_EXTENSIONS.rst

@@ -18,5 +18,7 @@ See the :manual:`cmake-compile-features(7)` manual for information on
 compile features and a list of supported compilers.
 
 This property is initialized by the value of
-the :variable:`CMAKE_OBJC_EXTENSIONS` variable if it is set when a target
-is created.
+the :variable:`CMAKE_OBJC_EXTENSIONS` variable if set when a target is created
+and otherwise by the value of
+:variable:`CMAKE_OBJC_EXTENSIONS_DEFAULT <CMAKE_<LANG>_EXTENSIONS_DEFAULT>`
+(see :policy:`CMP0128`).

Help/variable/CMAKE_LANG_EXTENSIONS.rst

@@ -11,7 +11,8 @@ The variations are:
 * :variable:`CMAKE_OBJCXX_EXTENSIONS`
 
 Default values for :prop_tgt:`<LANG>_EXTENSIONS` target properties if set when
-a target is created.
+a target is created.  For the compiler's default setting see
+:variable:`CMAKE_<LANG>_EXTENSIONS_DEFAULT`.
 
 For supported CMake versions see the respective pages.
 

Help/variable/CMAKE_LANG_EXTENSIONS_DEFAULT.rst

@@ -3,5 +3,8 @@ CMAKE_<LANG>_EXTENSIONS_DEFAULT
 
 .. versionadded:: 3.22
 
-Default value used when the :prop_tgt:`<LANG>_EXTENSIONS` target property is
-not set. Initialized during compiler detection to the compiler's default.
+Compiler's default extensions mode. Used as the default for the
+:prop_tgt:`<LANG>_EXTENSIONS` target property when
+:variable:`CMAKE_<LANG>_EXTENSIONS` is not set (see :policy:`CMP0128`).
+
+This variable is read-only.  Modifying it is undefined behavior.