SUNRPC: Clarify comment that documents svc_max_payload()

Note the function returns a per-transport value, not a per-request
value (eg, one that is related to the size of the available send or
receive buffer space).
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/svc.c

@@ -1554,8 +1554,12 @@ bc_svc_process(struct svc_serv *serv, struct rpc_rqst *req,
 EXPORT_SYMBOL_GPL(bc_svc_process);
 #endif /* CONFIG_SUNRPC_BACKCHANNEL */
 
-/*
- * Return (transport-specific) limit on the rpc payload.
+/**
+ * svc_max_payload - Return transport-specific limit on the RPC payload
+ * @rqstp: RPC transaction context
+ *
+ * Returns the maximum number of payload bytes the current transport
+ * allows.
  */
 u32 svc_max_payload(const struct svc_rqst *rqstp)
 {