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.