docs: say why the differences list exists, and record the emitter return values

[electrohyun]

Summary

Two changes to differences.md.

The page now says why it exists. Its value comes from how much else matches: a mock that is right almost everywhere gives a reader no reason to keep checking, and the reader who has stopped checking is the one a divergence reaches.

It also records a divergence that was not written down. socket.io returns the socket from socket.emit, socket.on, and the listener methods, and true from Server#emit and a broadcast operator's emit. smocket returns undefined in all of them, so socket.on('a', f).on('b', g) throws. This was found by installing the package from outside and using it, not by the suite, which never read a return value.

That entry goes in a new section C rather than in section A. Section A is titled "where smocket deliberately differs" and every entry there links to the decision behind it. This one is nobody's decision, and putting it in A would have broken the version rule as well as the heading: 0019 reads removing a section A entry as a major, so fixing the return values would have been judged a major purely because of which list the gap was written on, while the same fix judged on its own merits is a fidelity correction. Section C says what it holds, that its entries are expected to disappear, and which row governs closing one.

Section A is decisions, B is what smocket adds, C is what is simply missing. The three carry different version consequences, so they are three lists.

Related issue

No tracking issue. #189 removes the section C entry this adds, which is the case section C exists to keep off the section A row.

Checklist

• Tests added or updated. Documentation only.
• Behavior verified against real socket.io. Every return value named here was measured against a real server before being written down, and fix: return the socket from the client emitters and true from the server ones #189 carries the full table.

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Reviewing files that changed from the base of the PR and between 8e6e012 and 11b5ad6.

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• docs/differences.md

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