docs: note NFT position fee rounding

[leno23]

Summary

• Document the expected integer-rounding behavior in INonfungiblePositionManager fee accounting.
• Note that position updates can round down by less than 1 wei per token and that aggregate rounding loss may grow as a position is updated.

Closes #5
References Uniswap/v3-core#424

Test Plan

• npx --yes --package prettier@2.0.5 --package prettier-plugin-solidity@1.0.0-alpha.59 prettier --check contracts/interfaces/INonfungiblePositionManager.sol
• git diff --check