Generalize docpp and sphinx_ext

autotools/docpp
* handle generic custom directives in the form <class>_<kind>
* adapt handling of query fields

build/sphinx_ext.py
* add the BuildValuesDoc function to output definitions using the sphinx
  syntax that was already used for query fields
* adapt BuildQueryFields to use BuildValuesDoc

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>