[stdlib] Add a comment about use of Lemire's nearly divisionless method

[xwu]

In follow-up to #38161, make the clever recursion-breaking protocol implementation @_alwaysEmitIntoClient in addition to being unavailable.

Additionally, in follow-up to #25286, add a brief comment in the code about the use of Lemire's nearly divisionless method; the code is very neat but without any comment a reader who is unfamiliar with the algorithm has no easy way of understanding what's going on.

If accepted, this change should be nominated for inclusion in Swift 5.5 as per discussion in #38161.

[xwu]

@swift-ci smoke test

[stephentyrone]

Hah, saw this after I already opened #38272 and #38273

[xwu]

Heh, I threw the @_aEIC change in only because I was already editing the file. The principal purpose of this PR is to add the comment to help readers understand what’s going on. Happy to rebase if your merge goes through.

[xwu]

@stephentyrone Looks like it should merge cleanly without a rebase. OK to merge?