Correct static return type documentation

Description

There are several methods that document return types as CollectionInterface when they should be documented as static for IDE completion.

Motivation and context

By correcting these return types IDEs will understand the operations such as filter will return a collection of the same type.

How has this been tested?

I made the changes in my installed copy of ramsey/collection and verified that my IDE (PhpStorm) warnings went away.

I also ran composer run dev:test:all to verify nothing was broken.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

PR checklist

• [ ] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING.md document.
• [ ] I have added tests to cover my changes.