Help: Document CMAKE_OSX_SYSROOT default change in 4.0

In commit 7b19531291 (macOS: Do not pass any SDK/-isysroot to compilers
by default, 2024-11-06, v4.0.0-rc1~511^2) we forgot to update the
documentation of `CMAKE_OSX_SYSROOT`.

Fixes: #26846

Help/release/4.0.rst

@@ -232,8 +232,8 @@ Other Changes
 * Builds targeting macOS no longer choose any SDK or pass an ``-isysroot``
   flag to the compiler by default.  Instead, compilers are expected to
   choose a default macOS SDK on their own.  In order to use a compiler that
-  does not do this, users must now specify ``-DCMAKE_OSX_SYSROOT=macosx``
-  when configuring their build.
+  does not do this, users must now specify :variable:`CMAKE_OSX_SYSROOT`
+  when configuring their build, e.g., ``-DCMAKE_OSX_SYSROOT=macosx``.
 
 * On AIX, ``SHARED`` library targets now produce a shared library archive
   by default.  See policy :policy:`CMP0182`.

Help/variable/CMAKE_OSX_SYSROOT.rst

@@ -6,8 +6,13 @@ CMake uses this value to compute the value of the ``-isysroot`` flag
 or equivalent and to help the ``find_*`` commands locate files in
 the SDK.
 
-If not set explicitly the value is initialized by the ``SDKROOT``
-environment variable, if set, and otherwise computed based on the
-:variable:`CMAKE_OSX_DEPLOYMENT_TARGET` or the host platform.
+If not set explicitly, the value is initialized by the ``SDKROOT``
+environment variable, if set.  Otherwise, the value defaults to empty,
+so no explicit ``-isysroot`` flag is passed, and the compiler's default
+sysroot is used.
+
+.. versionchanged:: 4.0
+  The default is now empty.  Previously a default was computed based on
+  the :variable:`CMAKE_OSX_DEPLOYMENT_TARGET` or the host platform.
 
 .. include:: CMAKE_OSX_VARIABLE.txt