> - Something answering the question: "what's the difference between
> this and the existing 'alpha frame parameter",

I added paragraph after that for alpha with a brief description.
I have included it in the attached patch. For convenience, I'm
reciting the relevant part here:

====
   diff --git a/doc/lispref/frames.texi b/doc/lispref/frames.texi
   index 2eeb8b7ed7..05d0b1a72a 100644
   --- a/doc/lispref/frames.texi
   +++ b/doc/lispref/frames.texi
   @@ -2433,6 +2433,16 @@ Font and Color Parameters

    Some window systems do not support the @code{alpha} parameter for child
    frames (@pxref{Child Frames}).
   +
   +@vindex alpha-background@r{, a frame parameter}
   +@item alpha-background
   +@cindex opacity, frame
   +@cindex transparency, frame
   +Sets the background transparency of the frame. Unlike the @code{alpha}
   +frame parameter, this only controls the transparency of the background
   +while keeping foreground elements such as text fully opaque.
   +It should be an integer between 0 and 100, where 0 means
   +completely transparent and 100 means completely opaque (default).
    @end table

    The following frame parameters are semi-obsolete in that they are
====

Please let me know if this is what you had in mind or if I should revise
it.

> - a commit message containing a description of the changes in
> ChangeLog format

I have attached a file with a changelog entry. I'm also giving it here
for convenience:

====
   2022-01-23  Håkon Flatval  <hkon20@hotmail.com>
   	Add support for background transparency

   	* src/haikufns.c:
   	* src/nsfns.m:
   	* src/pgtkfns.c:
   	* src/w32fns.c:
   	* src/xfns.c: Add alpha-background frame parameter defaults. Prefer to use 32-bit depth visual in X.
   	* src/frame.h:
   	* src/frame.c: Add alpha-background frame parameter and setter function.
   	* src/gtkutil.c: Setup GTK to allow transparency.
   	* src/xterm.c: Draw background transparent using alpha-background frame parameter.
   	* doc/lispref/frames.texi: Add docs for alpha-background frame parameter
====

Let me know if it needs changes.

I'm a little confused as to where I should put this. Should I add it to
one of the Changelog files?

Best regards,
Håkon