Add motivation to "Augmentations" about code generation and FFI. (#4533)

Add motivation to "Augmentations" about code generation and FFI.

We usually think of augmentations as being generated code augmenting
hand-authored code. But there are use cases in the other direction as
well where the code generator produces the base declaration and a user
hand-writes an augmentation to tweak it.

Author: munificent