Documentation: changed tid parameter description for getThreadPriority()

Wrong description of the "tid" parameter used by getThreadPriority():
the function does not "change" the priority of the thread associated to 
"tid".
Defined also the meaning of setting tid parameter to 0.


Test: None

Change-Id: I8552f3b110eaab1aefa51477e82d3e295547be34
Signed-off-by: Alessio Balsini <alessio.balsini@gmail.com>