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

[Drew Adams <drew.adams@oracle.com>]

C-h f seq-concatenate
___

 seq-concatenate is a compiled Lisp function in 'seq.el'.

 (seq-concatenate TYPE SEQUENCE...)

 Concatenate SEQUENCES into a single sequence of type TYPE.
 TYPE must be one of following symbols: vector, string or list.


 This is a generic function.

 Implementations:

 (type &rest sequences) in 'seq.el'.

 Undocumented
___

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?


In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.18362
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''