Rewrote documentation for parse_bytes and to_str_bytes in {int, uint}_macros.rs
auto merge of #13792 : jacob-hegna/rust/master, r=alexcrichton

Just modified the documentation for parse_bytes to make it more clear how the bytes were parsed (big endian) and to show an example of what it returned.  I also added documentation for the to_str_bytes which previously had no documentation (besides one stackoverflow post).