bug#42323: 26.3; Doc string of `seq-map'

[Drew Adams]

FWIW, the notion of separation of interface from
implementation is not so clear for Lisp, and even
less so for Free Software Lisp.

Something implemented in "core" Emacs but in Lisp
is no more "the real thing" than the same thing
re-implemented by some user.  People can and do
reuse or modify "implementation" code.  And user
code sometimes does depend on how something is
"implemented".

In a way, take to an extreme, you could consider
the Lisp code distributed as part of GNU Emacs as
a "suggestion" to users.  (The C code as well,
but Lisp is particularly susceptible to, and even
invites, hacking, opportunistically or otherwise.)