Help: Document that target_sources defines [INTERFACE_]HEADER_SETS

Help/prop_tgt/HEADER_SETS.rst

@@ -8,5 +8,8 @@ all file sets with the type ``HEADERS``). Files listed in these file sets are
 treated as source files for the purpose of IDE integration. The files also
 have their :prop_sf:`HEADER_FILE_ONLY` property set to ``TRUE``.
 
+Header sets may be defined using the :command:`target_sources` command
+``FILE_SET`` option with type ``HEADERS``.
+
 See also :prop_tgt:`HEADER_SET_<NAME>`, :prop_tgt:`HEADER_SET` and
 :prop_tgt:`INTERFACE_HEADER_SETS`.

Help/prop_tgt/INTERFACE_HEADER_SETS.rst

@@ -8,4 +8,7 @@ all file sets with the type ``HEADERS``). Files listed in these header sets
 can be installed with :command:`install(TARGETS)` and exported with
 :command:`install(EXPORT)` and :command:`export`.
 
+Header sets may be defined using the :command:`target_sources` command
+``FILE_SET`` option with type ``HEADERS``.
+
 See also :prop_tgt:`HEADER_SETS`.

Help/release/3.23.rst

@@ -278,3 +278,7 @@ Changes made since CMake 3.23.0 include the following.
   targets.  Pending further work in a future version of CMake, it is now
   an error to add a ``FILE_SET`` of type ``HEADERS`` to such targets on
   Apple platforms.
+
+* The :prop_tgt:`HEADER_SETS` and :prop_tgt:`INTERFACE_HEADER_SETS` target
+  properties added in CMake 3.23.0 are now read-only records of the header
+  sets created by the :command:`target_sources` command.