add TTL section or desc

[yokofly]

timeplus database is a streaming database, for one simple row of new inserted data, we have a streaming store and a historical store.

got feedback about how to set the TTL(the user was confused about the logstore retention and TTL). is this intentional?
I find current docs are missing logstore_retention , we wanna hide this field or we are missing?

I think we need to add a section (prob 3 lines of wording) to explain:

• historical data TTL
• streaming data TTL