Explain empty mergeinfo in comments for svn:mergeinfo and svn_mergeinfo_t.
Explain empty mergeinfo in comments for svn:mergeinfo and svn_mergeinfo_t.

* subversion/include/svn_mergeinfo.h
  (global svn:mergienfo property comment): Explain empty mergeinfo and 
  update grammar.
  (svn_mergeinfo_t): Note how no mergeinfo and empty mergeinfo are 
  differentiated.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@871443 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,wbond/subversion