Document current Lua support

Anything that is not strictly callout/callin API and differs from expectations on vanilla Lua 5.1 development workflow (such as missing libs, require, etc) should be documented. Of main interest and highlight should be any actual patches to Lua 5.1 source code and changes (such as adding support for pairs via patch, etc).

Start by comparing the engine's current Lua 5.1 development workflow with vanilla Lua 5.1. Record differences outside the callout/callin API, including missing libraries and require behavior, and identify any patches to Lua 5.1 such as pairs support. Done means these differences and source changes are documented clearly.

Written by the indexing model from the issue text.