Preserve original comments around `url_preview_url_blacklist`

roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -1161,10 +1161,7 @@ url_preview_ip_range_whitelist: {{ matrix_synapse_url_preview_ip_range_whitelist
 # specified component matches for a given list item succeed, the URL is
 # blacklisted.
 #
-{% if matrix_synapse_url_preview_url_blacklist | length > 0 %}
-url_preview_url_blacklist: {{ matrix_synapse_url_preview_url_blacklist | to_json }}
-{% endif %}
-
+#url_preview_url_blacklist:
 #  # blacklist any URL with a username in its URI
 #  - username: '*'
 #
@@ -1181,6 +1178,9 @@ url_preview_url_blacklist: {{ matrix_synapse_url_preview_url_blacklist | to_json
 #
 #  # blacklist any URL with a literal IPv4 address
 #  - netloc: '^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$'
+{% if matrix_synapse_url_preview_url_blacklist | length > 0 %}
+url_preview_url_blacklist: {{ matrix_synapse_url_preview_url_blacklist | to_json }}
+{% endif %}
 
 # The largest allowed URL preview spidering size in bytes
 #