doc: add description to dirtytime_expire_seconds

commit 1efff914afac8a965ad63817ecf8861a927c2ace ("fs: add
dirtytime_expire_seconds sysctl") introduced dirtytime_expire_seconds
knob, but there is not description about it in
Documentation/sysctl/vm.txt.

Add the description for it.

Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sysctl/vm.txt

@@ -27,6 +27,7 @@ Currently, these files are in /proc/sys/vm:
 - dirty_bytes
 - dirty_expire_centisecs
 - dirty_ratio
+- dirtytime_expire_seconds
 - dirty_writeback_centisecs
 - drop_caches
 - extfrag_threshold
@@ -178,6 +179,18 @@ The total available memory is not equal to total system memory.
 
 ==============================================================
 
+dirtytime_expire_seconds
+
+When a lazytime inode is constantly having its pages dirtied, the inode with
+an updated timestamp will never get chance to be written out.  And, if the
+only thing that has happened on the file system is a dirtytime inode caused
+by an atime update, a worker will be scheduled to make sure that inode
+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.
+
+==============================================================
+
 dirty_writeback_centisecs
 
 The kernel flusher threads will periodically wake up and write `old' data