Document a channel mapping for 7 and 8 channel audio.

The FLAC spec hasn't previously documented channel
definitions for 7 and 8 channel files, making it
difficult to encode 6.1 an 7.1 audio. This makes
that definition.

The choice is based on popular surround configurations
for home theatre and gaming, using the Microsoft
WAVEX order, which most tools use internally.
Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>

format.html

@@ -1248,11 +1248,11 @@
 						<li>1 channel: mono</li>
 						<li>2 channels: left, right</li>
 						<li>3 channels: left, right, center</li>
-						<li>4 channels: left, right, back left, back right</li>
-						<li>5 channels: left, right, center, back/surround left, back/surround right</li>
-						<li>6 channels: left, right, center, LFE, back/surround left, back/surround right</li>
-						<li>7 channels: not defined</li>
-						<li>8 channels: not defined</li>
+						<li>4 channels: front left, front right, back left, back right</li>
+						<li>5 channels: front left, front right, front center, back/surround left, back/surround right</li>
+						<li>6 channels: front left, front right, front center, LFE, back/surround left, back/surround right</li>
+						<li>7 channels: front left, front right, front center, LFE, back center, side left, side right</li>
+						<li>8 channels: front left, front right, front center, LFE, back left, back right, side left, side right</li>
 					</ul>
 				</li>
 				<li>