Add `isInitCapture` and `forEachLambdaCapture` matchers.

This contributes follow-up work from https://reviews.llvm.org/D112491, which
allows for increased control over the matching of lambda captures. This also
updates the documentation for the `lambdaCapture` matcher.

Reviewed By: ymandel, aaron.ballman

Differential Revision: https://reviews.llvm.org/D113575