Clarify when ldisc->term may be NULL.

Clarify when ldisc->term may be NULL.

Namely, any ldisc that you send actual data through should have a
terminal attached, because the ldisc editing/echoing system is
designed entirely for use with a terminal. The only time you can have
an ldisc with no terminal is when it's only ever used by the backend
to report changes to the front end in edit/echo status, e.g. by Unix
Plink.

Coverity spotted an oddity in ldisc_send which after a while I decided
would never have actually caused a problem, but OTOH I agree that it
was confusing, so now hopefully it's less so.