Add some documentation describing the 'session URL' of an RA session.
Add some documentation describing the 'session URL' of an RA session.

* subversion/include/svn_ra.h
  (svn_ra_open4): Document that the given URL is remembered as the 'session
    URL'.
  (svn_ra_get_session_url, svn_ra_get_path_relative_to_session): Tweak
    wording relating to session URLs.

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