Document undocumented config options

[seanh]

There are 21 undocumented config options in CKAN, some of which are not mentioned in the config file template either:

ckan.admin.name ckan.admin.email ckan.default.group_type ckan.page_cache_enabled ckan.cache_enabled ckan.cache_expires ckan.extra_resource_fields ckan.extra_resource_group_fields ckan.storage.key_prefix ckan.storage.max_content_length ckan.feeds.authority_name * ckan.feeds.date * ckan.feeds.author_name * ckan.feeds.author_link * ckan.mail_from ckan.gravatar_default * ckan.plugins ckan.api_url ckan.auth.profile ckan.datastore.enabled ckan.tracking_enabled

There are also some options that are in the default deployment.ini even though they're deprecated:

ckan.async_notifier carrot_messaging_library ckan.build_search_index_synchronously

See email to ckan-dev from David Read: ​http://lists.okfn.org/pipermail/ckan-dev/2012-June/002447.html

It'd be best if the docs could be automatically pulled from the source into sphinx using autodoc, see #1358

[seanh]

Also see #2370