message: WIP doc managing activity logging context
diff: Asynchronous backend methods must cooperate to propagate a request's [ActivityId](../../overview/RpcInterface.md#logging-and-activityids) into logging messages. The current ActivityId is held by the [ActivityLoggingContext]($bentleyjs-core) class. The following rules must be followed by async methods to manage this class:
  ## Promise-returning functions
  -Every `Promise`-returning method *must*:
  +Every `Promise`-returning functions *must*:
  * Take an argument of type [ActivityLoggingContext]($bentleyjs-core).
  * Pass the input ActivityLoggingContext object to each Promise-returning method that it calls.
  * Call the [ActivityLoggingContext.enter]($bentleyjs-core) method on the input ActivityLoggingContext object:
  @@ -16,7 +16,9 @@ A Promise-returning function must *not* call ActivityLoggingContext.current.
  Note that a Promise-returning method is any method that returns a Promise, whether or not it is declared with the `async` keyword.
  -There is one exception to the above rule. An [RpcInterface implementation method](../RpcInterface.md#server-implementation) does not take a ActivityLoggingContext object as an argument. Instead, it must obtain the ActivityLoggingContext by calling [ActivityLoggingContext.current]($bentleyjs-core). An RcpInterface implementation method must follow all of the other the rules listed above. Other kinds of async functions must not call ActivityLoggingContext.current.
  +## RpcInterface implementation methods
  +
  +There is one exception to the above rule for Promise-returning functions. An [RpcInterface implementation method](../RpcInterface.md#server-implementation) does not take a ActivityLoggingContext object as an argument. Instead, it must obtain the ActivityLoggingContext by calling [ActivityLoggingContext.current]($bentleyjs-core). An RcpInterface implementation method must follow all of the other rules listed above.
  ## Callbacks to asynchronous functions
  Examples of asynchronous functions that invoke callbacks are:
  