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>

core/java/android/os/Process.java

@@ -740,7 +740,8 @@ public class Process {
     /**
      * Return the current priority of a thread, based on Linux priorities.
      * 
-     * @param tid The identifier of the thread/process to change.
+     * @param tid The identifier of the thread/process. If tid equals zero, the priority of the
+     * calling process/thread will be returned.
      * 
      * @return Returns the current priority, as a Linux priority level,
      * from -20 for highest scheduling priority to 19 for lowest scheduling