man: tc-pedit: Add examples for dsfield and retain

Describe a way to update just the DSCP and just the ECN part of the
dsfield. That is useful on its own, but also it shows how retain works.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David Ahern <dsahern@gmail.com>