getDoc() can produce invalid ReST when value_type is present

[jamadden]

Noticed in zope.configuration:

>>> from zope.configuration import config
>>> print(config.IDirectiveInfo['schema'].getDoc())
Directive handler

The dotted name of the directive handler

:Implementation: :class:`zope.configuration.config.DirectiveSchema`
:Read Only: False
:Required: True
:Default Value: None
.. rubric:: Value Type

:Implementation: :class:`zope.schema.InterfaceField`
:Read Only: False
:Required: True
:Default Value: None

Which produces a warning: (though the documentation is still printed):

$ sphinx-build -b html -d docs/_build/doctrees docs docs/_build/html
Running Sphinx v1.8.1
loading pickled environment... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 0 source files that are out of date
updating environment: [] 0 added, 2 changed, 0 removed
reading sources... [100%] api/zopeconfigure
docstring of zope.configuration.config.IDirectiveInfo.schema:9: WARNING: Field list ends without a blank line; unexpected unindent.