bug#37515: 27.0.50; button.el doesn't support help-echo function

[Lars Ingebrigtsen]

"Basil L. Contovounesios" <contovob@tcd.ie> writes:

> Some further changes are needed to fully support the help-echo property.
> Firstly, help-echo functions expect an overlay as their second argument
> when the object that had the property is an overlay (button).  Secondly,
> help-echo values can be forms in addition to strings and functions.
> Lastly, (info "(elisp) Button Properties") needs updating to reflect the
> supported values of the help-echo button property.
>
> The following patch should do all this.  I will push it in the next few
> days if there are no objections.  WDYT?

Looks good to me.

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