Remove code generating comments in configuration file (#12941)

docs/.sample_config_header.yaml

@@ -1,26 +1,12 @@
 # This file is maintained as an up-to-date snapshot of the default
-# homeserver.yaml configuration generated by Synapse.
-#
-# It is intended to act as a reference for the default configuration,
-# helping admins keep track of new options and other changes, and compare
-# their configs with the current default.  As such, many of the actual
-# config values shown are placeholders.
+# homeserver.yaml configuration generated by Synapse. You can find a
+# complete accounting of possible configuration options at
+# https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html
 #
 # It is *not* intended to be copied and used as the basis for a real
 # homeserver.yaml. Instead, if you are starting from scratch, please generate
 # a fresh config using Synapse by following the instructions in
 # https://matrix-org.github.io/synapse/latest/setup/installation.html.
-
-# Configuration options that take a time period can be set using a number
-# followed by a letter. Letters have the following meanings:
-# s = second
-# m = minute
-# h = hour
-# d = day
-# w = week
-# y = year
-# For example, setting redaction_retention_period: 5m would remove redacted
-# messages from the database after 5 minutes, rather than 5 months.
-
+#
 ################################################################################