Maint: Clarify doc string for contain function

The second sentence used "any given" in an ambiguous way. This edit clarifies
that only the classes provided as arguments will be declared.
Maint: Update and clarify docs for the include function

Include should mention contain and require. It should also give a brief primer
on the difference between include-like and resource-like class declarations.
Merge pull request #2073 from nfagerlund/contain_copyedit

`Contain` and `include` copyedits