Help: add notes for POSIX platforms to the DEFINE_SYMBOL target property

Fixes #24310

Help/prop_tgt/DEFINE_SYMBOL.rst

@@ -8,4 +8,10 @@ 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.
+properly setup dllexport/dllimport decorations on Windows.
+
+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.