Consider printing all qdoc warnings on the std::error stream, instead of the std::out stream.

Benefits

• Matches expectations: C++ compilers, python interpreters ... commonly print warnings on stderr, alongside error messages.
• Allows filtering: When launching qdoc through a build system, the stdout output usually contains information about the build step. Printing qdoc warnings on stderr allows for easier splitting of the build system output from the qdoc output.