tooltip-functions is a variable defined in `tooltip.el'.
Its value is
(tooltip-help-tips)
This variable may be risky if used as a file-local variable.
Documentation:
Functions to call to display tooltips.
Each function is called with one argument EVENT which is a copy
of the last mouse movement event that occurred. If one of these
functions displays the tooltip, it should return non-nil and the
rest are not called.