Add description of privileges

[x2048]

This PR adds a section about privileges to lua_api.txt, describing how to register and check for privileges and the meaning of the built-in privileges.

This PR is Ready for Review.

[Wuzzy2]

Isn't this like super redundant? And even we ignore that, I think this is better put in a separate document.

[lhofhansl]

@Wuzzy2 Redundant with what? Seems fine with me to add it here.

[Wuzzy2]

Redundant with /help privs

[Desour]

The names of all privs (and also chatcommands) defined by the engine (in C++ or builtin) should be listed, with a reference to the output of the help chatcommand, IMO.
Or, actually the ingame help text is not detailed enough, as it doesn't list everything that one can do with a priv, so they should be documented in detail somewhere, I don't care whether here or in another document.

(I didn't know (or forgot) that /help privs existed. Good that I do now, heh.)

This PR is missing some details about privileges in general. Ie.:

• Who can see what privs a player has?
• How far one can trust privs (ie. that players can be impersonated while they are on the server).
• Where the engine checks privs maybe. (Ie. whether a mod has to check the privs in a chatcommand def itself or if it's just for stylistic stuff (well, ok, that's more something for the chatcommand def doc).)
• Some information about the auth handler maybe.

(I'm probably requesting too much here, sorry.)

[x2048]

@Wuzzy2 The purpose here is to describe privileges as a feature in Minetest engine, just like e.g. drawtypes or texture modifiers.

I'll move the list of the built-in privileges to the end of the section, so that it does not cause confusion.

[Zughy]

@x2048 reminder about other sfan's comments to take care of 👁️

[x2048]

Not forgotten. Thanks :)