[#21634] Add comments on FieldValueGetter.

[mosche]

Follow up on #17172, adding some missing comments.

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[TheNeuralBit]

Could you clarify the meaning of "raw" here?

[mosche]

I've updated the comment @TheNeuralBit. For context, here's @reuvenlax's explanation of getValues.
From that it sounds like this shouldn't be publicly consumed and should also be annotated with @Internal.

getValues() is maybe poorly named - might be better called getRawValues. What you're looking for is probably the getBaseValues() method.
getValues is mostly used in code that knows exactly what it's doing for optimization purposes. It goes along with the attachValues method, which is similarly tricky to use. It's there to enable 0-copy code, but not necessarily intended for general consumption.