Update docs to clarify why forced builds may not be queued

If a project is already queued to build or actually building, an attempt to force a build does nothing.

It should either add another build to the queue or display a message saying why it is not doing so.

The current behavior is to not queue another build, and to log this message:
2010-01-12 17:31:23,638 [btpool0-9] INFO org.apache.continuum.buildmanager.ParallelBuildsManager - Project 1 is currently building in DEFAULT_BUILD_QUEUE

If the current behavior is kept, the log message should say that it is not queueing a build because it is already building.

The documentation on scheduled and forced builds needs to be updated with the intended behavior. I don't know if the current behavior is as designed, or a bug. http://continuum.apache.org/docs/1.3.4/user_guides/building_project/index.html