Intersphinx is a handy way of referencing external documentation, but it

has some limitations. Option directives cannot be looked up if using an

intersphinx prefix or a program name, severely limiting their usefulness

and reliability. It's also annoying to add a prefix to every reference

in a file, if wanting to ensure a specific set of documentation is being

used (like with release notes).
This change introduces a new Sphinx extension that backports a fix

targeting Sphinx 1.6.x for option references, ensuring that we can

referencing them properly today.
It also introduces a new directive for setting one or more specific

intersphinx names that should be used for any unresolved references. No

other intersphinx sets will be used when this is specified. This allows

a page to ensure the proper sets of documentation are linked.

Tested this with the RBTools release notes. Verified that all references

worked properly.