Improve doc strings in svn_string.h.
Improve doc strings in svn_string.h.

* subversion/include/svn_string.h
  (...everywhere...): Rewrite doc strings to make clear when input data
    is copied, describe all (except pool) parameters by name, and use
    consistent wording.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1415463 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion