ss: Add javadoc to StateSystem#waitUntilBuilt(long)

ss: Add javadoc to StateSystem#waitUntilBuilt(long)

If the timeout specified less or equal to 0, then there will no waiting and
this can be used as a "isBuilt" method for the state system.

Change-Id: I6110622071249cd456801fc220500740ba72d6b4
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/90309
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Alexandre Montplaisir <alexmonthy@efficios.com>