support/bundler: clarify handler calling behaviour
support/bundler: clarify handler calling behaviour

Clarify that the provided handler func is only called sequentially, and
that Flush waits for any outstanding calls to handler to have returned.

From the interface, it's not clear whether handlers are invoked in
parallel (in separate goroutines) or when an item is considered handled
(is it when the handler is enqueued or when it completes?).

Change-Id: Ia09c56319d6ac6b7f41fce682d6b00261721f1ff
Reviewed-on: https://code-review.googlesource.com/9570
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

google/google-api-go-client,googleapis/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client