docs: 5.Posting.rst: describe Fixes: and Link: tags

Explain Fixes: and Link: tags in Documentation/process/5.Posting.rst,
which are missing in this file for unknown reasons and only described in
Documentation/process/submitting-patches.rst.

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
CC: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Link: https://lore.kernel.org/r/c4a5f5e25fa84b26fd383bba6eafde4ab57c9de7.1641314856.git.linux@leemhuis.info
Signed-off-by: Jonathan Corbet <corbet@lwn.net>