ConcurrentHashMap: Add clarifying comments
ConcurrentHashMap: Add clarifying comments

Summary: Add comments about the performance of frequent removal and mention clear() in the comment about the timing of destruction of removed key-value pairs.

Reviewed By: davidtgoldblatt

Differential Revision: D21455366

fbshipit-source-id: be9f80136260f2de3b68d6fff54ccdadc5544046

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly