bug#7465: 24.0.50; transient-mark-mode customization UI is confusing

[Drew Adams]

> The complete text of the doc string is displayed, as usual, 
> but here it is confusing, because the values `lambda' and
> `(only . OLDVAL)' cannot be entered, since only toggling
> on-off is allowed.  As noted in the Elisp manual, these
> values are intended for Lisp programs, so they should not
> be mentioned in the Custom interface, which is concerned with
> user settings.

Users, even users of Customize, can also use Emacs Lisp.  The complete doc
string should always be available in Customize, and the Lisp interface should be
described in the doc string (not just the Customize interface).

However, you have a point in that the Lisp description is not identified as such
here, so this text can indeed be confusing to users.  The correct fix is to
introduce the description of the Lisp values by saying that these are the values
used in Lisp code.

Also, the text should not say that "non-nil _also_ enables highlighting...", for
this highlighting is a big part of what t-m mode is all about.  Please remove
"also" here.