Return the help-echo string at point.
Normally, the string produced by the `help-echo' text or overlay
property, or nil, is returned.
If KBD is non-nil, `kbd-help' is used instead, and any
`help-echo' property is ignored. In this case, the return value
can also be t, if that is the value of the `kbd-help' property. (fn &optional KBD)