rollup merge of #24350: tbelaire/cstring-doc-from-vec

It looks like `from_vec` was subsumed by new at some point,
but the documentation still refers to it as `from_vec`.

This updates the documentation for `from_vec_unchecked`
so that it properly says that it's the unchecked version of `new`.

Also, from_vec_unchecked requires a actual Vec<u8> while
new can take anything that is Into<Vec<u8>>, so I also
mention that in the documentation.

Since this is documentation:
r? @steveklabnik

diff --cc src/libstd/ffi/c_str.rs
Simple merge