Documentation: rxrpc: Demote three sections

Documentation: rxrpc: Demote three sections

Three sections ("Socket Options", "Security", and "Example Client Usage")
use title headings, which increase number of entries in the networking
docs toctree by three, and also make the rest of sections headed under
"Example Client Usage".

Demote these sections back to section headings.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: David Howells <dhowells@redhat.com>
Link: https://patch.msgid.link/20250922124137.5266-1-bagasdotme@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>