Audio description Understanding covers "important" visual content (#4324)

Two changes to the first note in the informative Understanding document:

- add "important" to match the first change to the definition note.
- swap out the word "provided" in preference for the synonym "conveyed"

The second change is made in the belief that "conveyed" is more
accurate. For example, if a scene shows something blowing up, and there
is the sound of an explosion that matches, it is more accurate to say
'the sound _conveys_ the visual explosion' than to say 'the sound
_provides_ the visual explosion'.

Author: mbgower

understanding/20/audio-description-or-media-alternative-prerecorded.html

@@ -57,8 +57,8 @@ <h2>Intent of Audio Description or Media Alternative (Prerecorded)</h2>
       <div class="note">
 
          <p>
-            For 1.2.3, 1.2.5, and 1.2.7, if all of the information in the video track is already
-            provided in the audio track, no audio description is necessary. 
+            For 1.2.3, 1.2.5, and 1.2.7, if all of the important information in the video track is already
+            conveyed in the audio track, no audio description is necessary. 
 
 
          </p>

understanding/20/audio-description-prerecorded.html

@@ -34,8 +34,8 @@ <h2>Intent of Audio Description (Prerecorded)</h2>
       <div class="note">
 
          <p>
-            For 1.2.3, 1.2.5, and 1.2.7, if all of the information in the video track is already
-            provided in the audio track, no audio description is necessary. 
+            For 1.2.3, 1.2.5, and 1.2.7, if all of the important information in the video track is already
+            conveyed in the audio track, no audio description is necessary. 
 
 
          </p>