Document the requirement on recomputed length for CString::from_raw (original) (raw)

A recent Doc PR for CStr reminded me of this:

CString::from_raw should make it clear that the length isn't just "recomputed," but that the recomputed length must match the original length. Yes, this can be inferred from the clearly-stated invariants of the type, but I feel this is important enough to deserve a sentence all of its own in the Unsafety section of the from_raw method, because it singlehandedly cripples a very wide range of would-be use cases for CString.
CString::into_raw should steer users away from using the pattern of CString::{into_raw,from_raw} when interfacing with C APIs that may change the effective length of the string by writing interior NULs or erasing the final NUL. (But what should we steer them towards? Vec<c_char>, probably? Hard to create one from string data though, compared to Vec<u8>...)