Document what we do with negative limits to our log functions.

* subversion/include/svn_repos.h
  (svn_repos_get_logs5): update docs to match behavior

* subversion/include/svn_ra.h
  (svn_ra_get_log3): ...

* subversion/include/svn_client.h
  (svn_client_log6): ...



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570335 13f79535-47bb-0310-9956-ffa450edef68