bug#32125: 26; Doc of `seqp` versus `sequencep`

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

Neither the doc strings nor the descriptions in (elisp) `Sequence
Functions' make clear what the difference is between these predicates.

(It's not even clear why `seq' was added.  What's that about?)

And the name `seqp' seems like a bad choice.  the predicate names
themselves should at least give some hint of the difference.

The doc for `seqp' should not just punt and tell users to go look in
`seq.el' to figure out what it means: "or any additional type of
sequence defined via 'seq.el' generic functions."

And in (elisp) `Sequence Functions' the entries for these two predicates
should be right next to each other.


In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''