docs: clarify that dirtytime_expire_seconds=0 disables writeback

Document that setting vm.dirtytime_expire_seconds to zero disables
periodic dirtytime writeback, matching the behavior of the related
dirty_writeback_centisecs sysctl which already documents this.

Signed-off-by: Laveesh Bansal <laveeshb@laveeshbansal.com>
Link: https://patch.msgid.link/20260106145059.543282-3-laveeshb@laveeshbansal.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: laveeshb

Documentation/admin-guide/sysctl/vm.rst

@@ -231,6 +231,8 @@ eventually gets pushed out to disk.  This tunable is used to define when dirty
 inode is old enough to be eligible for writeback by the kernel flusher threads.
 And, it is also used as the interval to wakeup dirtytime_writeback thread.
 
+Setting this to zero disables periodic dirtytime writeback.
+
 
 dirty_writeback_centisecs
 =========================