Call PF correctly from pcase--edebug-match-pat-args

Call PF correctly from pcase--edebug-match-pat-args

Also correct a doc string and insert commentary.  This fixes
bug#74052.

* lisp/emacs-lisp/edebug.el
(edebug--match-&-spec-op <&interpose>): Correct and complete
the doc string, which now says MUST call exactly once, and
documents the return values of FUN, PF and
edebug--match-&-spec-op.  Also remove an unneeded `(...)
construct.

* lisp/emacs-lisp/pcase.el (pcase--edebug-match-pat-args): Call
PF also for the main cases handled.

(cherry picked from commit fb5915665522f747daa8dfa11e91dc406d18edb7)