Document the Windows audio buffer size as 10 ms

After some double checking it turns out 20 ms is wrong - the buffer size
that the Windows Audio Engine uses internally is 10 ms (this is the size
of the buffer that APOs are called with, and it's also the "packet" size
it uses with WaveRT hardware).

20 ms may have been correct at some point in the distant past (Windows 7
maybe?), but it's clearly not anymore.

BACKENDS.md

@@ -69,8 +69,8 @@ pipeline. In particular, choice of backend can affect:
   backends.
 
 **Note:** the internal buffer size of the shared Windows audio pipeline has been
-observed to be 20 ms. This means that only exclusive backends (i.e. WASAPI
-Exclusive, WDM-KS) can achieve an actual latency below 20 ms.
+observed to be 10 ms. This means that only exclusive backends (i.e. WASAPI
+Exclusive, WDM-KS) can achieve an actual latency below 10 ms.
 
 **Note:** In addition to APOs, hardware devices can also implement additional
 audio processing at a low level in the audio driver, or baked into the hardware
@@ -99,7 +99,7 @@ the fact that MME appears to [behave very poorly][issue30] with small buffer
 sizes.
 
 Latency numbers reported by MME do not seem to take the Windows audio pipeline
-into account. This means the reported latency is underestimated by at least 20
+into account. This means the reported latency is underestimated by at least 10
 ms, if not more.
 
 The MME backend exposes "virtual" devices called *"Microsoft Sound Mapper -

FAQ.md

@@ -244,9 +244,6 @@ It depends on the [backend][backends]:
  - The default backend, **DirectSound**, as well as **MME**, are blind to most
    sources of latency (including the Windows audio pipeline itself), and can
    therefore grossly underestimate the actual latency.
-   - For example, both have been observed to report latencies below 20 ms, which
-     is not technically possible because that's the buffer size of the Windows
-     audio pipeline itself.
  - The **WASAPI** and **WDM-KS** backends have been observed to report more
    plausible latency numbers.