added documentation for R() (#69832)

* Updated docs/docsite/rst/dev_guide/developing_modules_documenting.rst, added documentation for R()
* added link to how to add anchors, anchor, and clarification on when to use each option

Co-authored-by: John R Barker <john@johnrbarker.com>
Co-authored-by: Alicia Cozine <879121+acozine@users.noreply.github.com>

docs/docsite/rst/dev_guide/developing_modules_documenting.rst

@@ -227,15 +227,18 @@ Linking within module documentation
 
 You can link from your module documentation to other module docs, other resources on docs.ansible.com, and resources elsewhere on the internet. The correct formats for these links are:
 
-* ``L()`` for Links with a heading. For example: ``See L(IOS Platform Options guide,../network/user_guide/platform_ios.html).``
+* ``L()`` for links with a heading. For example: ``See L(Ansible Tower,https://www.ansible.com/products/tower).`` As of Ansible 2.10, do not use ``L()`` for relative links between Ansible documentation and collection documentation.
 * ``U()`` for URLs. For example: ``See U(https://www.ansible.com/products/tower) for an overview.``
+* ``R()`` for cross-references with a heading (added in Ansible 2.10). For example: ``See R(Cisco IOS Platform Guide,ios_platform_options)``.  Use the RST anchor for the cross-reference. See :ref:`adding_anchors_rst` for details.
 * ``I()`` for option names. For example: ``Required if I(state=present).``
 * ``C()`` for files and option values. For example: ``If not set the environment variable C(ACME_PASSWORD) will be used.``
 * ``M()`` for module names. For example: ``See also M(win_copy) or M(win_template).``
 
 .. note::
 
-  For modules in a collection, you can only use ``L()`` and ``M()`` for content within that collection. Use ``U()`` to refer to content in a different collection.
+  For links between modules and documentation within a collection, you can use any of the options above. For links outside of your collection, use ``R()`` if available. Otherwise, use ``U()`` or ``L()`` with full URLs (not relative links). If you are creating your own documentation site, you will need to use the `intersphinx extension <https://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html>`_ to convert ``R()`` and ``M()`` to the correct links.
+
+
 
 .. note::
 

docs/docsite/rst/dev_guide/style_guide/index.rst

@@ -143,6 +143,8 @@ All internal links should use the ``:ref:`` syntax.
 Every page should have at least one anchor to support internal ``:ref:`` links.
 Long pages, or pages with multiple levels of headers, can also include a local TOC.
 
+.. _adding_anchors_rst:
+
 Adding anchors
 ^^^^^^^^^^^^^^