Sync FSX with FSFS:
Merge revisions r1603499, r1604903, r1605059, r1615348, r1617586 and
r1618153 from libsvn_fs_fs and resolve a few text conflicts.

These are just comment / docstring fixes.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635642 13f79535-47bb-0310-9956-ffa450edef68