pv-test, add comment as to why b:v is not being used for AV1 if only a bitrate is provided.

rc_shared.sh

@@ -1035,6 +1035,7 @@ function pv-test {
 									# Can be done like this if not changing over or under.
 									#bitrate="-b:v $vbr"
 
+									# Prefer this method so we know we're doing VBR (rc=1) not CBR (rc=2).
 									# rc=1, over defaults to 25, under defaults to 50
 									# rc=2, over defaults to 25, under defaults to 25
 									bitrate="-svtav1-params rc=1:tbr=${vbr}${o}${u}"