[Feature]: Passkey - Finishing checks and cleanup and documentation

[aramikm]

Feature Description

In this issue we would like to re-check all assumptions and done the finishing clean-up and adding left-over documentations

Acceptance Criteria

• Filter inner calls just like what we do for token accounts
• All modules and implementations should have concise and readable documents
• All combinations of transactions and validations should get checked to ensure there are no known leftover vulnerabilities
• Finishing cleanup and refactoring if required
• Readme matches the Pallet Readme template

[shannonwells]

Just putting this here as a reminder. In #2047, I noticed several validations in passkey/src/lib.rs that can be turned into one-liners, some if/elses that can be replaced with ensure!, etc.