BIP3: Address additional review

BIP3 got a bit more review from @RubenSomsen, @darosior, and @jonatack. Thanks!

This PR addresses the review. Most of it related to minor phrasing improvements, typos, and added emphasis.

I also added mention of the Version header and Changelog to the Backward Compatibility section which I had overlooked, and explain why I consider the headers "Superseded-by" and "Replaces" asymmetric, and only propose a new name for "Superseded-by" but retain "Replaces".

This PR includes a deletion and readdition of the entire BIP 3, so as to provide further reviewers a surface to leave comments directly on the BIP text. These two commits will disappear without a trace when the PR is later squash-merged.