[V1] Clarify input processing and multimodal feature caching logic

[ywang96]

As we open up V1 interface to community developers, IMO it's important to have a good level of documentation clarity in terms of code logic.

Since there has been good progress in #10114, this PR cleans up and adds clarifications on input processing and multimodal feature caching logic so that it's easier for new developers to get a high-level read without diving into the details of each component.

[github-actions]

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