Fix RT priority docs (NFC) (#3098)

The ranges and which value was specified as highest were incorrect on
some of them. On Linux, the range is 1 to 99 with 99 being highest.

From `man 7 sched`:
```
Processes scheduled under one of the real-time policies (SCHED_FIFO,
SCHED_RR) have a sched_priority value in the range 1 (low) to 99 (high).
```

Also clean up the relevant javadoc and doxygen comments.

hal/src/main/native/athena/Threads.cpp

@@ -32,8 +32,8 @@ int32_t HAL_GetThreadPriority(NativeThreadHandle handle, HAL_Bool* isRealTime,
     return sch.sched_priority;
   } else {
     *isRealTime = false;
-    // 0 is the only suppored priority for non-realtime, so scale to 1
-    return 1;
+    // 0 is the only supported priority for non-real-time
+    return 0;
   }
 }