MINOR: ring: permit to combine options on "show events"

MINOR: ring: permit to combine options on "show events"

The "show events" command supports a handful of options starting with
a "-", but only a few combinations were manually checked for (nw and
wn), other ones may not be combined.

This patch changes the parser to permit any combination, continuing to
process them in declaration order, so that -nsw or -rnw work. The doc
was updated.

This also updates the help message which would previously not list all
options by lack of room.