bug#47388: 27.1; eldoc-documentation-function is ignore in updated eldoc

[Alex Branham]

Hello -

Emacs 27.1 ships with eldoc that defines eldoc-documentation-function as
#'ignore. If users install eglot from ELPA, this pulls in an updated
eldoc that attempts to redefine eldoc-documentation-function. However,
the value does not change (since eldoc-documentation-function is already
bound, defcustom does not update it when called in
eldoc--documentation-strategy-defcustom) so the value is still ignore,
meaning users get:

    There is no ElDoc support in this buffer

since eldoc-documentation-function is #'ignore, regardless of the value
of eldoc-documentation-functions.

Here's the bug report we received with the original issue:
https://github.com/emacs-ess/ESS/issues/1130

Thanks!