DOC: add a diagram to explain how circular buffers work

DOC: add a diagram to explain how circular buffers work

Also add some thoughts about the existing and new design.

Note: an earlier design used the names "head" and "tail" for both sides
of the buffer, but it appears awkward as these words may be understood
in two forms (feed by head, output by tail, or make the newcomers wait
at the tail of the queue). Also there were already a few functions in the
code making use of either terminology. So better avoid this terminology
and use "input" and "output" instead.