[ERROR] Add to documentation that .flush() will be called automatically

[christy]

Note: This repository is ONLY used to solve issues related to DOCS.
For other issues, please move to other repositories.

Where is the error
What is the URL/path of the document related to this issue?
https://milvus.io/api-reference/pymilvus/v2.3.x/MilvusClient/Collections/describe_collection.md

How would you like to improve it
Make it clear that collection.flush() is automatically called before collection.describe().

For very large collections, this can incur a long wait. Which is unexpected, when user just thought they were calling .describe(). They expected that call to be fast, not slow!

Link to chat in our Discord: https://discord.com/channels/1160323594396635310/1216704641232928880/1216704641232928880

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