rust: pin-init: move proc-macro documentation into pin-init crate

Move the documentation of proc-macros from pin-init-internal into
pin-init. This is because documentation can only reference types from
dependencies and pin-init-internal cannot have pin-init as a dependency,
as that would be cyclic.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250308110339.2997091-5-benno.lossin@proton.me


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>