mm: reformat the Unevictable-LRU documentation

Do a bit of reformatting on the Unevictable-LRU documentation.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NLee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>