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

docs/differences.md

@@ -1,9 +1,16 @@
 # Differences from real socket.io
 
 > **TL;DR** The short list of where smocket and real socket.io do not line up: the
-> places smocket deliberately diverges (section A) and the API smocket adds that
-> socket.io has no equivalent for (section B). Each entry links to the decision
-> that explains it; the reasoning lives there, not here.
+> places smocket deliberately diverges (section A), the API smocket adds that
+> socket.io has no equivalent for (section B), and the gaps that are nobody's decision
+> and are waiting to be closed (section C). A section A entry links to the decision that
+> explains it; the reasoning lives there, not here.
+
+This page exists because of how much else matches. A mock that answers correctly almost
+everywhere gives a reader no reason to keep checking, and the reader who has stopped
+checking is the one a divergence reaches. The closer the fidelity gets, the more the
+remaining gaps depend on being written down. What to keep doubting is read here rather
+than discovered in a failing suite.
 
 ## A. Where smocket deliberately differs
 
@@ -50,3 +57,23 @@
   next tick. Order within the delayed stream is preserved, and scheduling runs through an
   injectable timer so a test drives it with fake timers rather than the wall clock. See
   [0018](./decisions/0018-delivery-scheduling-adapter-hook.md).
+
+## C. Known gaps, not deliberate, recorded until corrected
+
+Section A is where smocket chose to differ and section B is what it adds. This section is
+neither. These are places smocket does not match socket.io and no one decided that it
+should not, so they carry no decision record and are expected to disappear.
+
+The distinction is not only editorial. Removing a section A entry is a major under
+[0019](./decisions/0019-what-counts-as-a-breaking-change.md), because it withdraws a
+promise the project made on purpose. Closing one of these withdraws nothing. It is a
+correction toward measured real behaviour and takes that row instead, so the same fix does
+not change bump depending on which list it was written on.
+
+- **`emit` and the listener methods return nothing.** socket.io returns the socket from
+  `socket.emit(...)`, `socket.on(...)`, `socket.once(...)`, and the rest of the listener
+  methods, so the calls chain, while `Server#emit`, a namespace's `emit`, and a broadcast
+  operator's `emit` return `true`. smocket returns `undefined` in every one of those
+  positions, so `socket.on('a', f).on('b', g)` throws and a caller reading the result sees
+  a falsy value where socket.io gives a truthy one. Found by installing the package and
+  using it from outside rather than by the suite, which never read a return value.