Sync'ing FSX with FSFS:
Merge r1543594,1546100,1575418,1575430,1576427 from /subversion/libsvn_fs_fs
into subversion/libsvn_fs_x.  Conflicts were due earlier cherry-picks and
structural differences between FSX and FSFS.

This ports a documentation fixes.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1595346 13f79535-47bb-0310-9956-ffa450edef68