Follow-up to r1572640: update documentation.

* subversion/include/svn_config.h
  (SVN_CONFIG_REALMSTRING_KEY,
   SVN_CONFIG_AUTHN_USERNAME_KEY,
   SVN_CONFIG_AUTHN_PASSWORD_KEY,
   SVN_CONFIG_AUTHN_PASSPHRASE_KEY,
   SVN_CONFIG_AUTHN_PASSTYPE_KEY,
   SVN_CONFIG_AUTHN_ASCII_CERT_KEY,
   SVN_CONFIG_AUTHN_FAILURES_KEY): Split docstrings for doxygen.  Minor
     docstring updates. Move into a new 'cached_authentication_data_attributes'
     doxygen subgroup.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1572649 13f79535-47bb-0310-9956-ffa450edef68