[Documentation] Document existing APIs which are already in-place

[ivmarkov]

The following older sections of the code (pre 2024) need a lot more documentation:

• IM layer (partially). Stuff like the handlers, objects, encoding module and others
• SC layer - all of it
• Crypto code
• QR code
• ACLs
• Fabrics
• Maybe more?
• Sessions code in the transport layer, and possibly packet parsing / creation

I suggest we just add it over time. I can do IM layer and Sessions code as in the meantime my code contributions there prevail over the original code, and there, the complexity is a bit bigger.

I'll start opening PRs with this.