zsmalloc: document freeable stats

When freeable class stat was added to classes file (back in 2016) we
forgot to update zsmalloc documentation. Fix that.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 1120ed548394 ("mm/zsmalloc: add `freeable' column to pool stat")
Signed-off-by: Sergey Senozhatsky <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/mm/zsmalloc.rst

···
 	the number of pages allocated for the class
 pages_per_zspage
 	the number of 0-order pages to make a zspage
+freeable
+	the approximate number of pages class compaction can free
 
 Each zspage maintains inuse counter which keeps track of the number of
 objects stored in the zspage.  The inuse counter determines the zspage's