Documentation of ttl and exp on Catalog

[venkatramanm]

Describe the bug
On Catalog object returned as part of on_search, there are 2 fields ttl and exp. Not sure what is the difference between time to live and expiry in the same object. need clarification.

To Reproduce
Steps to reproduce the behavior:
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Expected behavior
Both ttl and exp need to be documented better or one of them removed.

Screenshots
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Desktop (please complete the following information):
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Additional context
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@ravi-prakash-v @rajaneeshk90