doc: mem-pool documentation

Change-Id: I9bc454c92a078de5e3a30372a50c6f2cbdcd6c21
BUG: 1118309
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/8343
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>