O11Y-1045: opentelemetry-dart must manage different contexts for asyn…

…c code

 - Improvements to Context documentation.

lib/src/api/context/context.dart

@@ -42,7 +42,8 @@ class Context {
   /// It should generally not be required to use the root [Context] directly -
   /// instead, use [Context.current] to operate on the current [Context].
   /// Only use this context if you are certain you need to disregard the
-  /// current [Context].
+  /// current [Context].  For example, when instrumenting an asynchronous
+  /// event handler which may fire while an unrelated [Context] is "current".
   static Context get root => Context._(Zone.root);
 
   /// Returns a key to be used to read and/or write values to a context.