Document that `read::GzDecoder` consumes bytes after end of gzip

[jongiddy]

Add tests showing that the GzDecoders in bufread and write support reading immediately after end of gzip data.

read::GzDecoder can read past the end of gzip data, and a subsequent read of the inner type loses those bytes.

[jongiddy]

I added a note to the read module describing the behaviour of read decoders and how to get the typically expected behaviour.