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 <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Acked-by: David Howells <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/networking/rxrpc.rst

···
 RXRPC_SUPPORTED_CMSG socket option (see below).
 
 
-==============
-SOCKET OPTIONS
+Socket Options
 ==============
 
 AF_RXRPC sockets support a few socket options at the SOL_RXRPC level:
···
      the highest control message type supported.
 
 
-========
-SECURITY
+Security
 ========
 
 Currently, only the kerberos 4 equivalent protocol has been implemented
···
 	http://people.redhat.com/~dhowells/rxrpc/listen.c
 
 
-====================
-EXAMPLE CLIENT USAGE
+Example Client Usage
 ====================
 
 A client would issue an operation by: