Fix the contradictory documentation of svn_mergeinfo_t and svn_mergeinfo_catalog_t.
Fix the contradictory documentation of svn_mergeinfo_t and
svn_mergeinfo_catalog_t.

* subversion/include/svn_mergeinfo.h
  (svn_mergeinfo_t,
   svn_mergeinfo_catalog_t): Be clear, mergeinfo keys are always absolute
   while mergeinfo catalog keys may be absolute or relative.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion