Overhaul pcase documentation

Overhaul pcase documentation

Suggested by Drew Adams (Bug#31311).

* doc/lispref/control.texi (Control Structures):
Add "Pattern-Matching Conditional" to menu, before "Iteration".
(Conditionals): Delete menu.
(Pattern matching case statement): Delete node/subsection,
by actually moving, renaming, and overhauling it to...
(Pattern-Matching Conditional): ...new node/section.
(pcase Macro): New node/subsection.
(Extending pcase): Likewise.
(Backquote Patterns): Likewise.
* doc/lispref/elisp.texi (Top) In @detailmenu, add
"Pattern-Matching Conditional" under "Control Structures"
section and delete "Conditionals" section.
* lisp/emacs-lisp/pcase.el (pcase): Rewrite docstring.
(pcase-defmacro \` (qpat) ...): Likewise.