CStr::from_bytes_with_nul_unchecked() incorrectly listed as const in docs

[nwn]

The documentation for CStr::from_bytes_with_nul_unchecked() lists it as a const fn, but its const-ness is still unstable (due to the rustc_const_unstable attribute and corroborated by the playground).

[RalfJung]

Cc @GuillaumeGomez
We have plenty of stable but const-unstable functions; are they all affected?

[GuillaumeGomez]

I guess? If they are under the same attribute, I see no reason for them to not share the same bug.

[GuillaumeGomez]

I wrote a fix. I just need to test a few things and then I send the PR.