Update doc/rados/configuration/filesystem-recommendations.rst

Clarified when it's necessary to use the setting:

filestore xattr use omap = true
Merge pull request #30 from rca/master

Minor clarification in docs.
doc: Removed the --without-tcmalloc flag until further advised.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
Merge git://github.com/ceph/ceph