Clarify which of the two non-blocking java connector implementations is selected by default.
Correct header in connectors comparison table (it labeled Nio[2] as blocking).

Partial backport of r1644331

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc8.0.x/trunk@1644336 13f79535-47bb-0310-9956-ffa450edef68