Explicitly say what sequence Mojo callbacks run on

It can be a point of confusion for newcomers.

BUG=

Change-Id: I27c854087e5978f4c89fb7ed10f1b2e852a4382a
Reviewed-on: https://chromium-review.googlesource.com/c/1382657
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Andrew Moylan <amoylan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617731}

mojo/public/cpp/bindings/README.md

@@ -327,7 +327,9 @@ class Logger {
 As before, both clients and implementations of this interface use the same
 signature for the `GetTail` method: implementations use the `callback` argument
 to *respond* to the request, while clients pass a `callback` argument to
-asynchronously `receive` the response. Here's an updated implementation:
+asynchronously `receive` the response. A client's `callback` runs on the same
+sequence on which they invoked `GetTail` (the sequence to which their `logger`
+is bound). Here's an updated implementation:
 
 ```cpp
 class LoggerImpl : public sample::mojom::Logger {