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: https://svn.apache.org/repos/asf/subversion/trunk@1548673 13f79535-47bb-0310-9956-ffa450edef68