rust: pin-init: re-enable doctests

The pin-init crate is now compiled in a standalone fashion, so revert
the earlier commit that disabled the doctests in pin-init in order to
avoid build errors while transitioning the crate into a standalone
version.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250308110339.2997091-22-benno.lossin@proton.me


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