fcntl.2: Add para introducing advisory locks and noting existence of OFD locks

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/fcntl.2

@@ -206,6 +206,11 @@ and
 .BR O_SYNC
 flags; see BUGS, below.
 .SS Advisory record locking
+Linux implements traditional ("process-associated") UNIX record locks,
+as standardized by POSIX.
+For a Linux-specific alternative with better semantics,
+see the discussion of open file description locks below.
+
 .BR F_SETLK ,
 .BR F_SETLKW ,
 and