Document the async use case with VertxHttpClientHTTPConduit

[ppalaga]

the lazy client initialization on first use may block if the WSDL URL is set via quarkus.cxf.client."client-name".wsdl. In that case the even loop would be blocked and the best practice is to suspend to a worker thread strainght away, e.g. by annotating the REST endpoint by @io.smallrye.common.annotation.Blocking
Edit: this is not needed anymore. We always dispatch the client to a worker thread (unless it is on a worker thread already) to solve #1685

Another general VertxHttpClientHTTPConduit topic: VertxHttpClientHTTPConduit does not honor default SunJSSE truststore - see #1665. Once we have quarkusio/quarkus#45175 it will be easier to set a CXT client to use $JAVA_HOME/lib/security/cacerts.