qapi: Make doc comments optional where we don't need them

qapi: Make doc comments optional where we don't need them

Since we added the documentation generator in commit 3313b61, doc
comments are mandatory.  That's a very good idea for a schema that
needs to be documented, but has proven to be annoying for testing.

Make doc comments optional again, but add a new directive

    { 'pragma': { 'doc-required': true } }

to let a QAPI schema require them.

Add test cases for the new pragma directive.  While there, plug a
minor hole in includ directive test coverage.

Require documentation in the schemas we actually want documented:
qapi-schema.json and qga/qapi-schema.json.

We could probably make qapi2texi.py cope with incomplete
documentation, but for now, simply make it refuse to run unless the
schema has 'doc-required': true.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1489582656-31133-3-git-send-email-armbru@redhat.com>
[qapi-code-gen.txt wording tweaked]
Reviewed-by: Eric Blake <eblake@redhat.com>