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

[Drew Adams <drew.adams@oracle.com>]

1. If you create a tooltip using, say, `x-show-tip', you can control the
font, text color and size, background color, etc.  Well, only some frame
parameters seem to have any effect, when passed in argument PARMS.  Just
which parameters can or cannot be used for PARMS should be documented.

2. The word `tooltip' is hardly mentioned in the doc.  Another doc fix
would be to document `tooltip-mode' properly in the Elisp manual, saying
which frame parameters you can use for `tooltip-frame-parameters', etc.

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).

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.

All of this kind of thing should be documented: what you can and cannot
do - which parameters have an effect.  You can make `help-echo' show an
image instead of text, but you cannot simply change the text appearance
of a `help-echo' string?  How are users supposed to guess what is
possible?  Should be documented.



In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2014-10-20 on LEG570
Bzr revision: 118168 rgm@gnu.org-20141020195941-icp42t8ttcnud09g
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking=yes,glyphs CPPFLAGS=-DGLYPH_DEBUG=1'