docs($rootScope): fix comment of Scope.$evalAsync

The current comment of Scope.$evalAsync contains two errors. First, it states that the expression will be executed on the current scope. In fact, the expression will be executed on whichever scope whose digest loop will run next. Second, it states that the function passed in this method will be invoked with the current scope being the first argument. However, the scope passed in is the scope whose digest loop is being run. The fix states the actual behaviors and warns users that the current scope might not be used for evaluation.