Use eldoc-documentation hook instead of variable

[mohkale]

Not sure which version of emacs added using hooks for eldoc instead of the eldoc variable so this may require bumping the minimum supported version.

Advantage of the new approach:

• Allows usage of multiple eldoc functions (for example flymake and ggtags).
• The above but also simultaneously. You can make eldoc show both lint and doc messages at the same time.

[leoliu]

eldoc-documentation-function is the preferred way for many releases including Emacs 27. eldoc-documentation-functions appeared at some point long long ago.

[mohkale]

@leoliu

Wow that's really recent. If that's the case then I can understand avoiding using the new hooks style until a few more emacs releases. I made this PR because I saw eldoc-message was deprecated in eldoc-1.1.0 meaning so was eldoc-documentation-function.

If you'd like you can close this issue. I'll probably re-open it when the need for this becomes more apparrent.

[leoliu]

There may be some recent change I am not aware of. will take a closer look later today. thanks for the PR :)

[leoliu]

I check the copy of eldoc.el on http://git.savannah.gnu.org/cgit/emacs.git/plain/lisp/emacs-lisp/eldoc.el and it seems there is incompatible changes likely to appear in emacs 28. Don't know why eldoc-documentation-function is removed instead of deprecated which will break many setups unnecessarily.

[mohkale]

@leoliu

Looking back on it you mention the hooks support has been in emacs for a while. Wouldn't that mean you can migrate ggtags to the hooks API and still support emacs's older than 27. Since eldoc-documentation-function is being removed and eldoc-documentation-functions is still available even on earlier releases, migrating seems like a win-win to me. Thoughts?