* lisp/simple.el (blink-paren-open-paren-line-string): Extract
functionality that shows the open paren line in the echo area into its
own function, to reuse it from paren.el.
(blink-matching-open): Use blink-paren-open-paren-line-string.
* lisp/paren.el (show-paren-context-when-offscreen): New option
show-paren-context-when-offscreen.
(show-paren-function): Implement it using
blink-paren-open-paren-line-string.
* lisp/emacs-lisp/eldoc.el (eldoc-display-message-no-interference-p):
Make sure the feature works well with eldoc.
* test/lisp/paren-tests.el (paren-tests-open-paren-line): Test
blink-paren-open-paren-line-string.
* doc/emacs/programs.texi (Matching): Update the documentation.
* etc/NEWS: And announce the new feature.
* lisp/paren.el: Use lexical-binding.
(show-paren--default): Use imperative form in doc-string.
* test/lisp/paren-tests.el: New file with tests for paren.el.
