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/source_group.rst

@@ -14,12 +14,16 @@ This is intended to set up file tabs in Visual Studio.
 The options are:
 
 ``TREE``
+ .. versionadded:: 3.8
+
  CMake will automatically detect, from ``<src>`` files paths, source groups
  it needs to create, to keep structure of source groups analogically to the
  actual files and directories structure in the project. Paths of ``<src>``
  files will be cut to be relative to ``<root>``.
 
 ``PREFIX``
+ .. versionadded:: 3.8
+
  Source group and files located directly in ``<root>`` path, will be placed
  in ``<prefix>`` source groups.
 
@@ -47,6 +51,9 @@ appropriately:
   source_group(outer\\inner ...)
   source_group(TREE <root> PREFIX sources\\inc ...)
 
+.. versionadded:: 3.18
+  Allow using forward slashes (``/``) to specify subgroups.
+
 For backwards compatibility, the short-hand signature
 
 .. code-block:: cmake