Rename Notifier::m_handlerMutex to Notifier::m_notifyMutex (#105)

A month ago, we discussed renaming this mutex to better reflect its current use, and this commit does that. The inline comment was also updated.

wpilibc/athena/include/Notifier.h

@@ -53,6 +53,6 @@ class Notifier : public ErrorBase {
   // true if this is a periodic event
   bool m_periodic = false;
 
-  // held by interrupt manager task while handler call is in progress
-  priority_mutex m_handlerMutex;
+  // held HAL notifier while Notifier::Notify() call is in progress
+  priority_mutex m_notifyMutex;
 };