Add collections to info about pagination support.

Currently: _Pagination does not support tags or categories_
Proposed: _Pagination does not support tags, categories or collections_

Maybe in the future? Related: #2376
Adding to pagination note: Cannot include collections

Adding a sentence to the paragraph to clarify, that pagination is restricted to posts and that it can't contain collections.
Merge pull request #2389 from kleinfreund/patch-1