Fix the documentation for svn_diff_hunk_readline_diff_text().
Fix the documentation for svn_diff_hunk_readline_diff_text().

This function has never returned the hunk header lines, despite several
significant refactorings.

* subversion/include/svn_diff.h
  (svn_diff_hunk_readline_diff_text): Correctly identify that the hunk header
    is not returned and direct users to the functions that can provide the
    data in the hunk header.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1548673 13f79535-47bb-0310-9956-ffa450edef68

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