[mojo docs] Clarify bidirectional communication with Mojo.

Typically, this is done with pairs of *interfaces*, not just pairs
of message pipes.

Change-Id: I4e83f45fda6b9666148bdd10a7c86ae727d41682
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1755407
Auto-Submit: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#687330}

mojo/docs/basics.md

@@ -83,10 +83,11 @@ And the other endpoint is the receiving/callee, represented as:
 mojo::Receiver<math::mojom::Math> receiver_;
 ```
 
-This allows limited bidirectional communication: the sender can make any number
-of calls to the receiver, and the receiver may send a single reply for each
-call. More expressive APIs (that require any number of calls to flow in both
-directions) are often implemented as a pair of message pipes.
+This allows limited bidirectional communication. For one interface, the sender
+(A) may make any number of calls to the receiver (B). (B) may send a single
+reply for each call from (A). More expressive APIs are often implemented as a
+pair of interfaces (with two underlying message pipes), allowing calls to be
+made in either direction between (A) and (B).
 
 Message pipe endpoints are typically created using one of: