Documentation of ttl and exp on Catalog

**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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**Additional context**
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@ravi-prakash-v @rajaneeshk90

Search the protocol specification for the Catalog object and its ttl and exp fields, then read their current definitions and usage in on_search. Clarify the semantic difference in the relevant documentation, or record the decision to remove one field; done means the specification unambiguously explains both fields or resolves the duplication.

Written by the indexing model from the issue text.