kernel-doc: Update "cross-referencing from rST" section to use automarkup

Update text and examples in the "Cross-referencing from
reStructuredText" section to reflect that no additional syntax is needed
anymore.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@protonmail.com>
Link: https://lore.kernel.org/r/20200903005747.3900333-3-nfraprado@protonmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>