Sync FSX with FSFS:  Merge revisions
r1623377, r1629879 and r1631049 from libsvn_fs_fs and resolve a
few text conflicts.

These are docstring fixes ported to FSX.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635965 13f79535-47bb-0310-9956-ffa450edef68