documentation/README: improve BitBake manual referencing guidelines

Removing the ":ref:`-D <bitbake:..." syntax as it's not necessary
Adding more and simpler ways of making references to the BitBake
manual.

(From yocto-docs rev: 7ad087a81e5dca7088c8088cd0cdcf31606baa6e)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Reviewed-by: Quentin Schulz <foss@0leil.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/README

@@ -328,10 +328,19 @@ The sphinx.ext.intersphinx extension is enabled by default
 so that we can cross reference content from other Sphinx based
 documentation projects, such as the BitBake manual.
 
-References to the bitbake manual can be done like this:
+References to the BitBake manual can be done:
+ - With a specific description instead of the section name:
+  :ref:`Azure Storage fetcher (az://) <bitbake:bitbake-user-manual/bitbake-user-manual-fetching:fetchers>`
+ - With the section name:
+  ":ref:`bitbake:bitbake-user-manual/bitbake-user-manual-intro:usage and syntax` option
+ - Linking to the entire BitBake manual:
+  :doc:`BitBake User Manual <bitbake:index>`
+
+Note that a reference to a variable (:term:`VARIABLE`) automatically points to
+the BitBake manual if the variable is not described in the Reference Manual's Variable Glossary.
+However, if you need to bypass this, you can explicitely refer to a description in the
+BitBake manual as follows:
 
-  See the ":ref:`-D <bitbake:bitbake-user-manual/bitbake-user-manual-intro:usage and syntax>`" option
-or
   :term:`bitbake:BB_NUMBER_PARSE_THREADS`
 
 Submitting documentation changes