mm, THP, doc: Add document for thp_swpout/thp_swpout_fallback

Add document for newly added thp_swpout, thp_swpout_fallback fields in
/proc/vmstat.

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/vm/transhuge.rst

@@ -355,6 +355,15 @@ thp_zero_page_alloc_failed
 	is incremented if kernel fails to allocate
 	huge zero page and falls back to using small pages.
 
+thp_swpout
+	is incremented every time a huge page is swapout in one
+	piece without splitting.
+
+thp_swpout_fallback
+	is incremented if a huge page has to be split before swapout.
+	Usually because failed to allocate some continuous swap space
+	for the huge page.
+
 As the system ages, allocating huge pages may be expensive as the
 system uses memory compaction to copy data around memory to free a
 huge page for use. There are some counters in ``/proc/vmstat`` to help