Add documentation for vendored_frameworks

[revolter]

The vendored_frameworks property is not documented at all. Nor is vendored_libraries or other vendored_* properties that I don't know about.

[segiddins]

https://guides.cocoapods.org/syntax/podspec.html#vendored_frameworks

[revolter]

Then it's an SEO problem that could be fixed.

[segiddins]

We’d appreciate a PR if you have any ideas