Enable warning messages in maven compiler plugin

"Show Warnings" is off by default in Maven Compiler Plugin. This invalidates our most recent setting of -Werror (treat warnings as errors).

Let's enable compiler warning messages in all projects and adjust Xdoclint setting:

<configuration>
  <showWarnings>true</showWarnings>
  <compilerArgs>
    ...
    <compilerArg>-Xdoclint:-missing</compilerArg>
    ...
 </compilerArgs>
</configuration>