bug#20540: 25.0.50; Document tooltip woes, including `help-echo'

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 9 May 2015 16:27:05 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> 
> 3. A related bug in `tooltip.el' is that the defcustom for
> `tooltip-frame-parameters' has no :type.  Impossible for a user to
> customize the value properly using Customize.  And no help about which
> parameters can be used effectively (see #2, above).

I cannot reproduce this one: my tooltip.el has this:

  (defcustom tooltip-frame-parameters
    '((name . "tooltip")
      (internal-border-width . 2)
      (border-width . 1))
    "Frame parameters used for tooltips.

  If `left' or `top' parameters are included, they specify the absolute
  position to pop up the tooltip.

  Note that font and color parameters are ignored, and the attributes
  of the `tooltip' face are used instead."
    :type 'sexp <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
    :group 'tooltip)

According to Git, this line was there for the last 18 years.

> 4. `help-echo': No doc saying whether the string can be propertized, and
> if so, which properties have any effect.  Although `x-show-tip' seems to
> let you change the char size, color, background color, etc., and you can
> use property `display' with `help-echo', apparently you cannot change
> the face attributes of the `help-echo' string so that the appearance
> changes.  This is quite a limitation, AFAICT.

See above: the doc string of tooltip-frame-parameters says font and
color parameters are ignored.