ElDoc: remember origin backend in doc snippets (bug#62029)

ElDoc: remember origin backend in doc snippets (bug#62029)

This lays groundwork for discriminating between different
documentation providers in ElDoc display outlets, i.e. members
of eldoc-display-functions

* lisp/emacs-lisp/eldoc.el (eldoc--make-callback): Take extra origin arg.
(eldoc-documentation-compose-1)
(eldoc-documentation-compose-eagerly)
(eldoc-documentation-default): Pass extra arg to eglot--make-callback.
(eldoc--invoke-strategy): Rework.
(eldoc-documentation-functions): Work docstring.