bug#42324: 26.3; Doc string of `seq-concatenate'

[Drew Adams]

> > The bug report is not about the implementation at
> > all.  It's about the doc specifying the behavior.
> 
> The doc string seems clear to me.  And since I don't understand what
> are #1, #2, and #3 in your notation, I cannot continue this discussion
> in any useful way, sorry.

>From the bug report (and elsewhere in the thread I
also made clear what #1, #2, and #3 referred to):

  1. See bug #42323 for the problem of the unhelpful,
     implicit reference to CL implementations and
     "generic function".

  2. The doc of `seq-concatenate' _really_ needs a
     description of how it differs from `cl-concatenate'.
     That's completely unclear.

  3. The doc says nothing about each SEQUENCE actually
     being automatically converted (by copying, presumably)
     into a real sequence: `seq-into-sequence'.  It's
     not clear what's allowed as SEQUENCE.

  #2 is the main reason I filed this bug report.  What's the difference?
  Why/when would you use one rather than the other?