chore: Add some documentation for the bls12-381 precompile constants

[kevaundray]

This PR adds some documentation for the BLS12-381 precompiles constants, in particular the ones related to the size of the inputs.

Currently in Draft because:

• I have added a few TODOs related to some constants that seem to not be needed
• I have added new lines between some constants and have not checked what is your preferred formatting rule
• I have made an opinionated change to put all of the gas related constants together, whereas before they were interleaved with the cryptography related constants

[codspeed-hq]

CodSpeed Performance Report

Merging #2222 will not alter performance

_{Comparing kevaundray:kw/constants-doc-bls120381 (1666016) with main (8c10ab1)}

Summary

✅ 8 untouched benchmarks