Improve the safety docs for `CStr`
Namely, the two functions `from_ptr` and `from_bytes_with_nul_unchecked`.
Before, these functions didn't state the requirements clearly enough,
and I was not immediately able to find them like for other functions.
This doesn't change the content of the docs, but simply rewords them for
clarity.
note: I'm not entirely sure about the '`ptr` must be valid for reads of `u8`.', there might be room for improvement for this (and maybe for the other docs as well 😄)
a9c4a7e1aa