Merge #21300: script: Add explanatory comment to tc.sh

3a0446fad4 script: Add explanatory comment to tc.sh (dscotese)

Pull request description:

  This is a replacement for #21289

  tc.sh is used to limit bandwidth. I ran it and it is limiting my bandwidth. When I ran it, I got one error. I have not found an explanation anywhere of what the error means, but my best guess is consistent with the result, so I propose the explanatory comment to save others time when they use it and also get the error.

ACKs for top commit:
  laanwj:
    that said, LGTM ACK 3a0446fad4

Tree-SHA512: 5403a2a0fec3724625c20402a96334c3c7a620324a930c5fd828017da8911d2867aecb7a2ad94a23d1f189009d3eb197a67eb59c8e4531fd215d9b1edb600440

contrib/qos/tc.sh

@@ -16,7 +16,7 @@ LOCALNET_V4="192.168.0.0/16"
 #defines the IPv6 address space for which you wish to disable rate limiting
 LOCALNET_V6="fe80::/10"
 
-#delete existing rules
+#delete existing rules ('Error: Cannot delete qdisc with handle of zero.' means there weren't any.)
 tc qdisc del dev ${IF} root
 
 #add root class