; Improve doc strings in seq.el

; Improve doc strings in seq.el

* lisp/emacs-lisp/seq.el (seq-doseq, seq-setq, seq-elt)
(seq-length, seq-rest, seq-do, seq-mapn, seq-drop, seq-take)
(seq-drop-while, seq-take-while, seq-sort, seq-sort-by)
(seq-concatenate, seq-into, seq-filter, seq-remove)
(seq-remove-at-position, seq-every-p, seq-some, seq-find)
(seq-count, seq-contains, seq-contains-p, seq-set-equal-p)
(seq-position, seq-positions, seq-uniq, seq-mapcat)
(seq-partition, seq-union, seq-intersection, seq-difference)
(seq--count-successive, seq--elt-safe, seq-random-elt)
(seq-split, seq-keep): Doc fixes.