chore: improve comments accuracy

[0xChin]

Small PR improving the accuracy of the comments related to the gas required for the FeesDepositor. Here is a Gist with the traces of the gas costs when FeesDepositor receives ETH and triggers/doesn't trigger a portal deposit: https://gist.github.com/0xChin/f0e05c23599b9966f5601d27f71f5aae

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Note

Clarifies L1Withdrawer.sol docs to state the L1 gas limit for FeesDepositor must be at or above 800k gas (constructor and setter comments).

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