Document log format configs in valkey.conf (#1233)

Add config options for log format and timestamp format introduced by
#1022
Related to #1225

This change adds two new configs into valkey.conf:
log-format
log-timestamp-format

---------

Signed-off-by: azuredream <zhaozixuan67@gmail.com>

valkey.conf

@@ -348,6 +348,23 @@ pidfile /var/run/valkey_6379.pid
 # nothing (nothing is logged)
 loglevel notice
 
+# Specify the logging format.
+# This can be one of:
+#
+# - legacy: the default, traditional log format
+# - logfmt: a structured log format; see https://www.brandur.org/logfmt
+#
+# log-format legacy
+
+# Specify the timestamp format used in logs using 'log-timestamp-format'.
+#
+# - legacy: default format
+# - iso8601: ISO 8601 extended date and time with time zone, on the form
+#   yyyy-mm-ddThh:mm:ss.sss±hh:mm
+# - milliseconds: milliseconds since the epoch
+#
+# log-timestamp-format legacy
+
 # Specify the log file name. Also the empty string can be used to force
 # the server to log on the standard output. Note that if you use standard
 # output for logging but daemonize, logs will be sent to /dev/null