Help: Note when <project>_COMPAT_VERSION were added

Tweak documentation of project `_COMPAT_VERSION` variables to note when
they were added. Note that, while these are "experimental" for  now,
they are always set; the experimental gate is only required for them to
be set to non-empty values (and since they are optional, they may be
empty regardless).

Help/variable/CMAKE_PROJECT_COMPAT_VERSION.rst

@@ -1,6 +1,8 @@
 CMAKE_PROJECT_COMPAT_VERSION
 ----------------------------
 
+.. versionadded:: 4.1
+
 .. note::
 
   Experimental. Gated by ``CMAKE_EXPERIMENTAL_EXPORT_PACKAGE_INFO``.

Help/variable/PROJECT-NAME_COMPAT_VERSION.rst

@@ -1,6 +1,8 @@
 <PROJECT-NAME>_COMPAT_VERSION
 -----------------------------
 
+.. versionadded:: 4.1
+
 .. note::
 
   Experimental. Gated by ``CMAKE_EXPERIMENTAL_EXPORT_PACKAGE_INFO``.

Help/variable/PROJECT_COMPAT_VERSION.rst

@@ -1,6 +1,8 @@
 PROJECT_COMPAT_VERSION
 ----------------------
 
+.. versionadded:: 4.1
+
 .. note::
 
   Experimental. Gated by ``CMAKE_EXPERIMENTAL_EXPORT_PACKAGE_INFO``.