Improve Electric Pair mode documentation (bug#78021)

Improve Electric Pair mode documentation (bug#78021)

* doc/emacs/programs.texi (Matching): Clarify and improve
documentation of Electric Pair mode.

* lisp/elec-pair.el: Improve description in header line.  Add text
and a reference to the Emacs user manual in the Commentary section.
(electric-pair-skip-self, electric-pair-inhibit-predicate)
(electric-pair-preserve-balance)
(electric-pair-delete-adjacent-pairs)
(electric-pair-open-newline-between-pairs)
(electric-pair-skip-whitespace)
(electric-pair-skip-whitespace-function)
(electric-pair-analyze-conversion)
(electric-pair--skip-whitespace)
(electric-pair-text-syntax-table, electric-pair--with-syntax)
(electric-pair-syntax-info, electric-pair--insert)
(electric-pair--syntax-ppss, electric-pair--balance-info)
(electric-pair-inhibit-if-helps-balance)
(electric-pair-skip-if-helps-balance)
(electric-pair-open-newline-between-pairs-psif)
(electric-pair-mode): Clarify and improve doc strings and some comments.
(electric-pair-post-self-insert-function): Restructure doc string
to shorten overlong first line, and reformat overlong lines of code.

(cherry picked from commit bb735331650a9b5ac3c28ed8e1d9459059bd1195)