CLEANUP: backend: fix incorrect comments on locking conditions for lb functions

The leastconn and roundrobin functions mention that the server's lock
must be held while this is not true at all and it is not used either.
The "first" algo doesn't mention anything about the need for locking,
so let's mention that it uses the lbprm lock.
diff --git a/src/lb_fas.c b/src/lb_fas.c
index 9f6dc24..fb581eb 100644
--- a/src/lb_fas.c
+++ b/src/lb_fas.c
@@ -292,7 +292,7 @@
 /* Return next server from the FS tree in backend <p>. If the tree is empty,
  * return NULL. Saturated servers are skipped.
  *
- * The server's lock must be held. The lbprm's lock will be used.
+ * The lbprm's lock will be used. The server's lock is not used.
  */
 struct server *fas_get_next_server(struct proxy *p, struct server *srvtoavoid)
 {