Improve server docstrings for the cache size parameters.  In particular,
make it clear that '0 membuffer caches' does not mean no caches at all.

* subversion/mod_dav_svn/mod_dav_svn.c
  (cmds): Clarify what happens if SVNInMemoryCacheSize is set to 0.

* subversion/svnserve/svnserve.c
  (svnserve__options): Add description of '-M 0' behavior.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1644035 13f79535-47bb-0310-9956-ffa450edef68