Improve texture modifier documentation

[Wuzzy2]

This PR updates the texture modifier documentation by making it somewhat more accurate and precise.

Work based on https://docs.luanti.org/for-creators/api/texture-modifiers/, the texts are basically copied from there with minor changes. Thanks to @appgurueu.

(Ideally, it would have been better if those changes would have gone to lua_api.md first, but … oh well)

To do

• Wait for Document integer data types and ranges in Lua API #16735 to get merged
• Update texture mofidier documentation to correctly document integer ranges, following the spirit of Document integer data types and ranges in Lua API #16735

How to test

Read the changes. ;-)

[j-r]

I think this affects other operations like [invert, too. Perhaps make this a general comment with a suitable link to e.g. https://en.wikipedia.org/wiki/Alpha_compositing (not sure if that is a good resource, though).

[Wuzzy2]

I think the documentation already implies that texture modifiers "stack" so I don't see a need to update.

[j-r]

I might be missing something, but I think [noalpha is not the only modifier capable of changing pixel alpha from zero to a non zero value. IIUC [invert can do it, too, if mode contains a. And those modifiers (not sure if there are more) could benefit from a similar comment.

[Wuzzy2]

Addressed your comments.
I also completely rewrote the [crack documentation because it turns out that [crack behaves much differently than we previously believed (determined by testing).

So my recommended use of [crack is the 2-parameter version both for normal and animated textures. The 3-parameter version seems rather esoteric because it creates a grid of cracks. I suppose this could be used for tilesheets.