Docs: Gas Differences

[Stebalien]

We need to document a bunch of differences in how gas works:

1. Precompile gas limits don't work. That includes the 1/64th rule.
2. The ratio of Filecoin to EVM gas is 222:1 purely in terms of chain bandwidth. However, opcodes have completely different prices.
3. EVM gas is not a thing in Filecoin.
4. FEVM explicitly makes no guarantees on gas values. The current gas values will change and any contracts with hard-coded gas limits will break on upgrades.

[raulk]

EVM gas is not a thing in Filecoin.
FEVM explicitly makes no guarantees on gas values. The current gas values will change and any contracts with hard-coded gas limits will break on upgrades.

These were already documented in FIP-0054. Adding (1) and (2) now.

[Stebalien]

Additionally, invalid opcodes do not use the remaining gas.