MINOR: ring/cli: improve the "show events" parser regarding options

MINOR: ring/cli: improve the "show events" parser regarding options

The "show events" parser remains counter-intuitive in that the options
are historically placed after the ring name, making it behave differently
from commonly used tools and other CLI commands.

Let's relax the parser to take any -XXX as a set of option and the only
word not starting with - as the sink name. That way it becomes possible
to parse options before or after the sink name. The doc was updated to
reflect this and present available options as an enumeration rather than
an unreadable wall of text.