Help: Add documentation for `<target>_EXPORTS` compile definition

Fixes: #27554

Help/manual/cmake-buildsystem.7.rst

@@ -404,9 +404,14 @@ for compiling a target.
   These are passed to the compiler with ``-D`` flags, or equivalent,
   in an unspecified order.
 
-  The :prop_tgt:`DEFINE_SYMBOL` target property is also used
-  as a compile definition as a special convenience case for
-  ``SHARED`` and ``MODULE`` library targets.
+  When compiling sources of a ``SHARED`` library, a ``MODULE`` library,
+  or an ``EXECUTABLE`` with :prop_tgt:`ENABLE_EXPORTS` enabled, CMake
+  automatically defines a target-specific preprocessor symbol.
+  By default the definition is of the form ``<target>_EXPORTS``, but it
+  can be overridden by the :prop_tgt:`DEFINE_SYMBOL` target property.
+  This allows headers to detect whether they are included from inside
+  their implementation sources, and to correctly set up export/import
+  annotations or visibility of symbols.
 
 :prop_tgt:`COMPILE_OPTIONS`
   List of compile options for compiling sources in the target.

Help/prop_tgt/DEFINE_SYMBOL.rst

@@ -1,17 +1,21 @@
 DEFINE_SYMBOL
 -------------
 
-Define a symbol when compiling this target's sources.
+Define a preprocessor symbol when compiling this target's sources.
 
-``DEFINE_SYMBOL`` sets the name of the preprocessor symbol defined when
-compiling sources in a shared library.  If not set here then it is set
-to ``target_EXPORTS`` by default (with some substitutions if the target is
-not a valid C identifier).  This is useful for headers to know whether
-they are being included from inside their library or outside to
-properly setup dllexport/dllimport decorations on Windows.
+CMake adds this definition when compiling sources of a ``SHARED`` library,
+a ``MODULE`` library, or an ``EXECUTABLE`` with :prop_tgt:`ENABLE_EXPORTS`
+enabled.  If ``DEFINE_SYMBOL`` is not set, the default definition is of
+the form ``<target>_EXPORTS`` (with some substitutions if the target is
+not a valid C identifier).
+
+The symbol is only defined while compiling the target itself and is not
+propagated to dependent targets.
 
 On POSIX platforms, this can optionally be used to control the visibility
 of symbols.
 
 CMake provides support for such decorations with the :module:`GenerateExportHeader`
 module.
+
+See also the ``COMPILE_DEFINITIONS`` under :ref:`Target Compile Properties`.