printk: console_msg_format was added in v4.16

Author: cirosantilli

README.adoc

@@ -409,7 +409,7 @@ where:
 
 This format is selected by the following boot options:
 
-* `console_msg_format=syslog`: add the `<LEVEL>` part
+* `console_msg_format=syslog`: add the `<LEVEL>` part. Added in v4.16.
 * `printk.time=y`: add the `[TIMESTAMP]` part
 
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