Clarify unit symbols

[gerhard]

RabbitMQ supports both power of 2 & power of 10 unit symbols. Giving clear examples of the difference - it's a mistake easy to make.

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• 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)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)

Checklist

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• I have read the CONTRIBUTING.md document
• 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

Further Comments

Pushed this commit into master first, by mistake, had to revert.

[michaelklishin]

We have @dcorbacho's approval since we are pairing :)

[michaelklishin]

Cherry-picked to v3.7.x.