Notate points where the implementation could commit

[domenic]

As seen in https://chromium-review.googlesource.com/c/chromium/src/+/1648902 by @inexorabletash, it can be a nice optimization for implementations to commit various transactions at certain points.

Although this has no observable consequences to consumer code, it still seems like a worthwhile thing for the spec to note. I'm thinking non-normative notes explaining that inserting a commit after this step would potentially speed things up.