docs: Document bakedInput

[jemgillam]

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.

[jemgillam]

https://github.com/graphile/crystal/blob/main/.changeset/tricky-boxes-tickle.md