Add Haddock in PlutusTx.Builtins.Internal

[zliu41]

This module is for directly invoking UPLC builtin functions from Plinth. There needs to be Haddock explaining what each builtin function does (some of which are very non-obvious), so that users won't have to dig into the Plutus Core spec.

[kwxm]

Note this big comment about the four integer division operations. That has somehow ended up in PlutusTx.Ratio, but it should at least be referred to in Builtins.Internal.

[Unisay]

Do we expect Plinth users to directly import functions from this module?