chore: Add explainer comment to each permission type caveats.ts

[jeffsmale90]

Description

It's critically important that the caveats are encoded correctly. This PR adds an explainer comment to the caveats file outlining the terms encoding, and how they relate to the abi encoding of the allowed function.

Pre-merge author checklist

• I've followed MetaMask 7715 Permissions Snaps Contributor Docs and MetaMask 7715 Permissions Snaps Coding Standards.
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

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Note

Low Risk
Documentation-only updates in caveat builders; no changes to term construction or enforcement logic, so runtime behavior should be unchanged.

Overview
Adds/expands in-file JSDoc explainers across ERC-20 and native-token permission caveat builders (periodic, streaming, and ERC-20 allowance revocation) to clarify what each enforcer restricts.

For erc20TokenRevocation, documents the ABI calldata layout for approve(address,uint256) and how AllowedCalldataEnforcer uses startIndex to pin the selector and zero amount, plus comments clarifying ValueLteEnforcer usage.

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