bug#59167: 27.2; eldoc-documentation-strategy being 'ignore prevents new eldoc supports

[daanturo]

This version takes `eldoc-documentation-strategy`'s `default-value` to set
instead (the previous one may take the buffer-local value).

Another justification for the patch:

If `eldoc-documentation-strategy` is already set before loading the new eldoc,
loading the library will pop this warning up:

```
Warning (defvaralias): Overwriting value of ‘eldoc-documentation-strategy’ by 
aliasing to ‘eldoc-documentation-function’
```

By setting `eldoc-documentation-function` before defining
`eldoc-documentation-strategy` as alias, we can preserver its value of avoid
this warning.


By the way, why was `eldoc-documentation-strategy` made obsolete instead of
`eldoc-documentation-function`, isn't the latter's usage ough to be warned
against?

The second patch declares the correct obsolete one.

-- 
Daanturo.

0002-Eldoc-on-old-Emacs-correct-the-obsolete-variable.patch
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0001-Don-t-set-eldoc-strategy-to-ignore-on-Emacs-27-and-b.patch
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