Chunked type / documentation cleanup

[natecook1000]

This change improves the documentation for evenlyChunked(in:) and chunks(ofCount:) and renames EvenChunksCollection to EvenlyChunkedCollection to follow the pattern of wrapper type names matching their producing functions.

Checklist

• [x] I've added at least one test that validates that my change is working, if appropriate
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test