Clarify ReadBuf::uninit allows initialized buffers as well

[zacknewman]

Motivation

The documentation for ReadBuf::uninit makes it seem as though the passed buffer must be fully uninitialized; however it allows any buffer, a subset of which is fully uninitialized memory.

Note this is more for people like me who are deficient in unsafe code; thus the original documentation is likely obvious for those who are well acquainted with unsafe code.

Solution

State the buffer may be fully uninitialized.