Small cleanup of outdated docs

[HEnquist]

I found two things in the documentation that look like they could use an update.
Firstly, the readme says:

If you just want direct access to the unsafe bindings, use [coreaudio-sys](https://crates.io/crates/coreaudio-sys).

Should this not recommend the objc2 crates instead?

The second thing is this in lib.rs

//! Currently, work has only been started on the [audio_unit](./audio_unit/index.html) module, but
//! eventually we'd like to cover at least the majority of the C API.

This was written 10 years ago and audio_unit is still the only module, so the second part there seems a bit unrealistic. How about cutting it down to something like just "This library currently covers Audio Unit (with a suitable link)."?

Makes sense? I could make a PR with some suggested changes.

[simlay]

Should this not recommend the objc2 crates instead?

Yes.

Makes sense? I could make a PR with some suggested changes.

They would definitely be welcome!

[HEnquist]

See the PR :)
On a side note, I just noticed that clippy gives a bunch of warnings. 15 are harmless and easy to fix, but these two looks like maybe something:

error: this public function might dereference a raw pointer but is not marked `unsafe`
   --> src/audio_unit/mod.rs:394:50
    |
394 |     unsafe { try_os_status!(AudioUnitSetProperty(au, id, scope, elem, data_ptr, size)) }
    |                                                  ^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#not_unsafe_ptr_arg_deref
    = note: `#[deny(clippy::not_unsafe_ptr_arg_deref)]` on by default

error: this public function might dereference a raw pointer but is not marked `unsafe`
   --> src/audio_unit/mod.rs:423:13
    |
423 |             au, id, scope, elem, data_ptr, size_ptr
    |             ^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#not_unsafe_ptr_arg_deref

Another PR?

[simlay]

Another PR?

Sure! Tech Debt cleanup is always welcome!