rxrpc: kdoc: Update function descriptions and add link from rxrpc.rst

rxrpc: kdoc: Update function descriptions and add link from rxrpc.rst

Update the kerneldoc function descriptions to add "Return:" sections for
AF_RXRPC exported functions that have return values to stop the kdoc
builder from throwing warnings.

Also add links from the rxrpc.rst API doc to add a function API reference
at the end.  (Note that the API doc really needs updating, but that's
beyond this patchset).

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20250411095303.2316168-2-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>