General Oblique Transformation doc: add "Rosetta Stone" linking it to *merc

[jidanni]

On
https://proj.org/en/9.3/operations/projections/omerc.html
there is the "Rosetta Stone"

Example: Verify that the Mercator, and Transverse Mercator ... are limiting forms of the Oblique Mercator

Please add a similar "Rosetta Stone" to
https://proj.org/en/9.3/operations/projections/ob_tran.html

Something like

Example: Verify that the Mercator, and Transverse Mercator, and
Oblique Mercator are limiting forms of the General Oblique Transformation

I would write it, except, well, I am grasping to understand it in the
first place.

Indeed the Mercator family are just one of several families that could be used for an example, but at least it is a good starter.

Thanks!

[Jeitan]

Wow. Yes please. I am trying to use the General Oblique transformation to make a rotated projection based on an Oblique Mercator, but I haven't been able to even reproduce that basic case, and I just stumbled across this request which would help TREMENDOUSLY. I can't even reproduce a normal Transverse Mercator at 0 longitude 0 latitude with no rotation.

ETA: Okay, so I can produce a normal Transverse Mercator a 0,0, by "moving" the pole using o_lat_p and o_lon_p. But moving the pole is not ideal for my particular use-case, and when I try to use o_lon_c and o_lat_c and alpha .... strange things happen. I can get it right-side up using alpha = 90 but I have no idea how it's deciding on the center (using 0, 0 puts the origin at latitude=0 and longitude = +90). All the above just to illustrate the need for what this request is asking for.