Correct Chunked documentation

[mdznr]

• Changed “same” to “equal” to clarify that this is an equality (Equatable, ==/!=) comparison, not identify comparison (===/!==).
• Corrected algorithmic complexity documentation in Chunked.md
• Added parameter documentation for belongInSameGroup and projection

Checklist

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

[mdznr]

There definitely is a difference in complexity between the eager and lazy functions, but I’m finding it difficult to describe succinctly.

The eager functions are O(n) where n is the number of elements in the collection.

The lazy functions on the order of O(n) since the start index could require iterating to the very end of the collection. Each iteration of the receiving lazy sequence is also on the order of O(n), but total n where n is the number of elements in the collection—not n².

[natecook1000]

@swift-ci Please test