Documentation/filesystems: describe the shared memory usage/accounting

The Shared Memory accounting support is present in Kernel since commit
4b02108ac1 ("mm: oom analysis: add shmem vmstat") and in userland
free(1) since 2014.  This patch updates the Documentation to reflect
this change.

Signed-off-by: Rodrigo Freire <rfreire@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/proc.txt

@@ -855,6 +855,7 @@ Dirty:             968 kB
 Writeback:           0 kB
 AnonPages:      861800 kB
 Mapped:         280372 kB
+Shmem:             644 kB
 Slab:           284364 kB
 SReclaimable:   159856 kB
 SUnreclaim:     124508 kB
@@ -911,6 +912,7 @@ MemAvailable: An estimate of how much memory is available for starting new
    AnonPages: Non-file backed pages mapped into userspace page tables
 AnonHugePages: Non-file backed huge pages mapped into userspace page tables
       Mapped: files which have been mmaped, such as libraries
+       Shmem: Total memory used by shared memory (shmem) and tmpfs
         Slab: in-kernel data structures cache
 SReclaimable: Part of Slab, that might be reclaimed, such as caches
   SUnreclaim: Part of Slab, that cannot be reclaimed on memory pressure

Documentation/filesystems/tmpfs.txt

@@ -17,10 +17,10 @@ RAM, where you have to create an ordinary filesystem on top. Ramdisks
 cannot swap and you do not have the possibility to resize them. 
 
 Since tmpfs lives completely in the page cache and on swap, all tmpfs
-pages currently in memory will show up as cached. It will not show up
-as shared or something like that. Further on you can check the actual
-RAM+swap use of a tmpfs instance with df(1) and du(1).
-
+pages will be shown as "Shmem" in /proc/meminfo and "Shared" in
+free(1). Notice that these counters also include shared memory
+(shmem, see ipcs(1)). The most reliable way to get the count is
+using df(1) and du(1).
 
 tmpfs has the following uses: