Improve documentation of several functions in seq.el

Improve documentation of several functions in seq.el

* doc/lispref/sequences.texi (Sequence Functions):
* lisp/emacs-lisp/seq.el (seq-contains):
(seq-contains-p):
(seq-set-equal-p):
(seq-position):
(seq-union):
(seq-intersection):
(seq-difference): Use more standard wording in the docstrings
(bug#57561).