Add documentation for singular vs plural form of properties

[revolter]

This was (and it still is) very confusing for me when I created my first Pod. There are examples that use both of them, and I can't tell the difference if any. Nor do I understand how it works, as, for example, there is no non-string weak_framework instance in the codebase, but I'm using it and it works.

[segiddins]

We'd need to beef up the docs in CocoaPods/Core, it comes from https://sourcegraph.com/github.com/CocoaPods/Core/-/blob/lib/cocoapods-core/specification/dsl.rb#L752 (the singularize: true bit)