Document limitations of refraction from using screen texture

[tetrapod00]

Closes #90171.

The existing note in the manual is long and I don't think it is appropriate to copy it entirely, or even to list all of these caveats in a single note in the class reference:

Refraction is implemented as a screen-space effect and forces the material to be transparent. This makes the effect relatively fast, but this results in some limitations:
Transparency sorting issues may occur.
The refractive material cannot refract onto itself, or onto other transparent materials. A refractive material behind another transparent material will be invisible.
Off-screen objects cannot appear in the refraction. This is most noticeable with high refraction strength values.
Opaque materials in front of the refractive material will appear to have "refracted" edges, even though they shouldn't.

We can expand this note over time, if any other issues can be closed by adding to it.

[Repiteo]

Thanks!