Docs: include propsData in mounting options

[sagalbot]

What problem does this feature solve?

While migrating my project from tests that I wrote while Vue was in V1, I experienced a bit of friction figuring out how to use shallowMount while setting prop data. It took me a bit to track down propsData, as I'd never actually needed it in an application setting.

I understand that propsData is actually handled through the extends API, but it seems like such a common use case for this library that documentation would be beneficial.

What does the proposed API look like?

I think it would be beneficial to have a propsData link included under the 'Mounting Options' doc. It could be explained within there that this is actually the responsibility of extends API and not vue-test-utils.

I'm happy to PR a change to the docs with info about propsData.

[eddyerburgh]

Yes good idea, please make a PR as you suggested :)