llm-cache: document the public APIs

[sighingnow]

Document the semantic, arguments, return values, as well as requirements for certain argument (e.g., whether the memory should be allocated by the caller).

[github-actions[bot]]

/cc @dashanji @vegetableysm, this issus/pr has had no activity for for a long time, could you folks help to review the status ? To suppress further notifications,

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