Improve documentation of destructuring-binding macros

Improve documentation of destructuring-binding macros

* lisp/emacs-lisp/pcase.el (pcase-dolist, pcase-let)
(pcase-let*): Improve the doc strings.

* doc/lispref/sequences.texi (Sequence Functions): Improve
wording and rename arguments of seq-let to be more
descriptive.  Add a cross-reference to "Destructuring with
pcase Patterns".
* doc/lispref/control.texi (Pattern-Matching Conditional):
Improve wording and the menu.
(pcase Macro): Incorporate patch suggested by Paul Eggert
<eggert@cs.ucla.edu>.  Reformat text.
(Destructuring with pcase Patterns): Rename from
"Destructuring patterns", and improve wording and indexing.