mime: add examples for FormatMediaType and ParseMediaType
Change-Id: Ic129c58784ad1f0b8b90fc9d33e52bee61bdf0eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/253237
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>