Help: Add .. versionadded directives to commands documentation

This change ony concerns directives that appear in the document body.

The guidelines for inserting version directives:

 * Baseline version is CMake 3.0, i.e. directives start at 3.1.
 * Always use `.. versionadded::` directive, avoid ad-hoc version
   references. Exception: policy pages.
 * For new command signatures, put `versionadded` on a separate line
   after the signature.
 * For a group of new signatures in a new document section,
   a single version note at the beginning of the section is sufficient.
 * For new options, put `versionadded` on a separate line before
   option description.
 * If all the option descriptions in the list are short one-liners,
   it's fine to put `versionadded` on the same line as the description.
 * If multiple option descriptions in close proximity would have
   the same ..versionadded directive, consider adding a single
   directive after the list, mentioning all added options.
 * For compact value lists and sub-option lists, put a single
  `versionadded` directive after the list mentioning all additions.
 * When a change is described in a single paragraph, put
   `versionadded` into that paragraph.
 * When only part of the paragraph has changed, separate the changed
   part if it doesn't break the flow. Otherwise, write a follow-up
   clarification paragraph and apply version directive to that.
 * When multiple version directives are close by, order earlier
   additions before later additions.
 * Indent related lists and code blocks to include them in the scope
   of `versionadded` directive.

Issue: #19715

Help/command/export.rst

@@ -64,16 +64,23 @@ build tree. In some cases, for example for packaging and for system
 wide installations, it is not desirable to write the user package
 registry.
 
-By default the ``export(PACKAGE)`` command does nothing (see policy
-:policy:`CMP0090`) because populating the user package registry has effects
-outside the source and build trees.  Set the
-:variable:`CMAKE_EXPORT_PACKAGE_REGISTRY` variable to add build directories to
-the CMake user package registry.
+.. versionadded:: 3.1
+  If the :variable:`CMAKE_EXPORT_NO_PACKAGE_REGISTRY` variable
+  is enabled, the ``export(PACKAGE)`` command will do nothing.
+
+.. versionadded:: 3.15
+  By default the ``export(PACKAGE)`` command does nothing (see policy
+  :policy:`CMP0090`) because populating the user package registry has effects
+  outside the source and build trees.  Set the
+  :variable:`CMAKE_EXPORT_PACKAGE_REGISTRY` variable to add build directories
+  to the CMake user package registry.
 
 .. code-block:: cmake
 
   export(TARGETS [target1 [target2 [...]]]  [ANDROID_MK <filename>])
 
+.. versionadded:: 3.7
+
 This signature exports cmake built targets to the android ndk build system
 by creating an Android.mk file that references the prebuilt targets. The
 Android NDK supports the use of prebuilt libraries, both static and shared.