Fix doc issues with newer Doxygen and Sphinx

Fix doc issues with newer Doxygen and Sphinx

In krb5.hin, fix some apparently harmless whitespace nits in comments
which cause newer Doxygen versions to generate <detaileddescription>
content containing <linebreak/> tags.  Also remove the explicit "@n"
linebreaks in the responder JSON schemas, instead relying on Doxygen
markup support (added in Doxygen 1.8).  Add a verbatim handler in
doxybuilder_types.py to translate the resulting section, replacing the
linebreak handler which is no longer needed.

css_files is deprecated in Sphinx.  Instead use app.add_stylesheet()
in a setup function in conf.py.  (Sphinx 1.8 has a slightly simpler
method, but that is currently too recent to depend on.)  Also remove
the obsolete html_use_smartypants setting.

In func_document.tmpl, fix an incorrent use of "is".

ticket: 8952 (new)