Clarify Unix Time units in AmqpTimestamp

[ngbrown]

Proposed Changes

This improves the documentation of AmqpTimestamp to specify that Unix Time is in seconds. This is in the AMQP 0.9.1 spec at section 4,2.5.4, but having it in the code documentation makes it nicer to work with.

Types of Changes

• [ ] Bug fix (non-breaking change which fixes issue #NNNN)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• [x] Documentation improvements (corrections, new content, etc)
• [ ] Cosmetic change (whitespace, formatting, etc)

Checklist

• [x] I have read the CONTRIBUTING.md document
• [x] I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• [ ] All tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)
• [ ] Any dependent changes have been merged and published in related repositories

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