[patch] better explain $target.h vs $target-protos.h in internals manual

[Richard Sandiford]

Sandra Loosemore <sandra@codesourcery.com> writes:
> Following up on discussion in gcc@, how does this documentation patch look?
>
> https://gcc.gnu.org/ml/gcc/2018-02/msg00139.html

Looks good to me FWIW.

Richard