Avoid missing docs warning on pub extern crate

[jannic]

Due to rust-lang/rust#112308, the public re-export of the pac will trigger a missing docs warning in rust 1.71.0.

This is bogus, as the doc string would not be shown in the rendered docs anyway. However, the warning would be annoying, so add a short doc string to the item.

[ithinuel]

Does the issue arise if we use edition 2021's pub use rp2040_pac as pac; ?

[jannic]

Does the issue arise if we use edition 2021's pub use rp2040_pac as pac; ?

It doesn't seem so, but it leads to lots of other errors:

    Checking rp2040-hal v0.9.0-alpha.1 (/home/jan/rp2040/rp-rs/rp-hal/rp2040-hal)
error[E0433]: failed to resolve: use of undeclared crate or module `pac`
 --> rp2040-hal/src/i2c/controller.rs:6:5
  |
6 | use pac::{i2c0::RegisterBlock as Block, RESETS};
  |     ^^^ use of undeclared crate or module `pac`

error[E0433]: failed to resolve: use of undeclared crate or module `pac`
 --> rp2040-hal/src/pwm/reg.rs:2:5
  |
2 | use pac::pwm::CH;
  |     ^^^ use of undeclared crate or module `pac`

[...]
error: could not compile `rp2040-hal` (lib) due to 255 previous errors; 4 warnings emitted

Probably all fixable by adjusting those use statements, but is it worth the effort?

[ithinuel]

Since our MSRV has edition 2021 support anyway, do we have any benefits from sticking with 2018?

[jannic]

I don't think so. It's probably just that nobody did the conversion, because it doesn't give us significant benefits either.

Maybe https://doc.rust-lang.org/stable/edition-guide/rust-2021/panic-macro-consistency.html could add some additional overhead because panic! now always involves string formatting. But my gut feeling is that, as we already have panic statements which use string formatting, the overhead will be very small, if any.