docs: correct indication of complete key from "partial"

[aawilson]

Reserve_ids indicates complete_key in its typespec and in its type-checking, but says "Partial" in the description after the typespec. I also believe "use as a base for reserved IDs" is ambiguous, but I'm not familiar enough with the underlying API to know a better description since I'm not 100% sure on the behavior is supposed to be.

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Fixes #37 🦕

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[aawilson]

@googlebot I signed it!

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[tseaver]

Thanks very much for the patch!