Break the quantile documentation into a summary-specific paragraph
Break the quantile documentation into a summary-specific paragraph

to untangle the summary and histogram configuration, give an explicit
example of a summary match.

Signed-off-by: Matthias Rampke <8e7be411ad89ade93d144531f3925d0bb4011004@soundcloud.com>

prometheus/statsd_bridge,prometheus/statsd_exporter