Thank you for working on this, Yamaoka-san.

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?