make `#[pg_guard]` less internally confusing

[workingjubilee]

I had a talk with @eeeebbbbrrrr about FFI stuff in the context of #1661.

During that conversation, we made mistakes/were confused on which of the two major functions (pg_guard_ffi_boundary and pgrx_extern_c_guard) are used for which case (decorating an extern "C" fn rust_callable_by_c(Args) -> Ret { expr } and decorating an extern "C" { fn c_callable_by_rust(c_type) -> c_type; } binding declaration) like three times in that conversation. The reason it was only three was that I was talking slowly because I was confused and still only halfway through my second coffee.

We need to make sure which-is-which is eye-bleedingly clear to ourselves. At minimum, via documentation (which could also cover #1406).

[workingjubilee]

We also discussed splitting the macro into two parts, so it's harder to confuse ourselves, but the internal structure when-to-use-what being clear is more important than the external API.