pub fn introduce()Expand description
Print the whole of auth login --help’s guidance, once, before anything is
asked for.
It used to appear a block at a time, above the prompt each block belonged
to. That is too late: by the time the token prompt asks for a token, the
person has already gone looking for one. Both blocks up front means the
procedure can be followed from the top, in one place, without leaving the
command — and it is the same text --help prints, so neither can drift into
being the real instructions while the other rots.