DOC: internal: add a bit of documentation for the stconn closing conditions

DOC: internal: add a bit of documentation for the stconn closing conditions

The conditions where ERR, EOS and EOI are found are not always
crystal clear, and the fact that there's still a good bunch of
original ones dating from the early days and that seem to test for
non-existing cases doesn't help either.

After auditing the code base and projecting the 3 main muxes' stream
termination conditions, with Christopher and Amaury we could establish
the current flags matrix which indicates both what each combination
means for each mux and when it is set by each of them (or not set and
for what reason).

It should be sufficient to void doubts when adding code or when chasing
a bug.

It *must not* be backported because it is highly specific to the latest
2.8-dev.