docs: Document bakedInput
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From a beta2 changeset: 

> `FieldArgs.get` is no more; use `FieldArgs.getRaw` or use `bakedInput()` (TODO: document) to get the "baked" version of a raw input value.

> Input object fields no longer have `applyPlan`/`inputPlan`, instead having the
runtime equivalents `apply()` and `baked()`. `FieldArgs` is no longer available
on input object fields, since these fields are no longer called at plantime;
instead, the actual value is passed.

