Improve help messages about multi-language mode support

As in http://docs.codehaus.org/display/SONAR/Analysis+Parameters#AnalysisParameters-ProjectConfiguration

>`sonar.language`
>Set the language of the source code to analyze. Browse the Plugin Library page to get the list of all available languages. If not set, a multi-language analysis will be triggered.
 
>`sonar.sources`
>Comma-separated paths to directories containing source files.
Compatible with Maven since SonarQube 4.2. If not set, the source code is retrieved from the default Maven source code location. 
argsBuilder.append already check that value is not blank before adding the param. So this change is useless.