Simplify code and improve doc strings.

* subversion/include/svn_mergeinfo.h
  (svn_rangelist_inheritable2,
   svn_mergeinfo_inheritable2): Improve doc strings.

* subversion/libsvn_subr/mergeinfo.c
  (svn_rangelist_inheritable2): Simplify code. Fix comments.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1569815 13f79535-47bb-0310-9956-ffa450edef68