Added link to explanation of why --trust-sender is required

Author: davidjarzebowski

backup.config-example

@@ -39,7 +39,7 @@ GHE_NUM_SNAPSHOTS=10
 #GHE_EXTRA_SSH_OPTS=""
 
 # Any extra options passed to the rsync command.
-# As of April 2023 the --trust-sender flag is required. 
+# As of April 2023 the --trust-sender flag is required. See docs/requirements.md#update-april-2023 for details on this change.
 GHE_EXTRA_RSYNC_OPTS="--trust-sender"