Fix #1879: ABSN playback algorithm offset

basically move the phrase from the paragraph into the second item,
with a few minor changes for grammar and flow.

Added a link to computedPlaybackRate.

index.bs

@@ -4897,15 +4897,15 @@ Playback of AudioBuffer Contents</h4>
 
 This normative section specifies the playback of the contents of
 the buffer, accounting for the fact that playback is influenced by
-the following factors working in combination, which can vary
-dynamically during playback:
+the following factors working in combination:
 
 * A starting offset, which can expressed with sub-sample precision.
 
-* Loop points, which can expressed with sub-sample precision.
+* Loop points, which can expressed with sub-sample precision and can
+	 vary dynamically during playback.
 
 * Playback rate and detuning parameters, which combine to yield a
-	single computed playback rate that can assume non-infinite values
+	single <a>computedPlaybackRate</a> that can assume finite values
 	which may be positive or negative.
 
 The algorithm to be followed internally to generate output from an