bytes: add documentation to reader methods

Some methods that were used to implement various `io` interfaces in the
Reader were documented, whereas others were not. This change adds
documentation to all the missing methods used to implement these
interfaces.

Change-Id: I2dac6e328542de3cd87e89510651cd6ba74a7b7d
Reviewed-on: https://go-review.googlesource.com/65231


Reviewed-by: Ian Lance Taylor <iant@golang.org>