settings: Add doveadm_config_version

data/settings.js

@@ -3490,6 +3490,20 @@ many connections to the doveadm service.
 If \`0\`, commands are run directly in the same process.`
 	},
 
+	doveadm_config_version: {
+		values: setting_types.STRING,
+		text: `
+Dovecot configuration version. This must be the first setting in the
+configuration file. It specifies the configuration syntax, the used setting
+names and the expected default values.
+
+When there are default configuration changes in newer Dovecot versions, the
+existing installations will continue to work the same as before with the same
+default settings until this version number is increased. If there are other
+configuration changes, the old configuration will either keep working or there
+will be a clear failure at startup.`
+	},
+
 	dsync_alt_char: {
 		default: '_',
 		tags: [ 'dsync' ],

docs/core/settings/syntax.md

@@ -22,6 +22,12 @@ Dovecot doesn't care which settings are in which files. They could all be put
 into a single `dovecot.conf` if preferred (except for
 [external config files](#external-config-files)).
 
+## Config Version
+
+The first setting in the configuration file must be
+[[setting,dovecot_config_version]]. It specifies the configuration syntax,
+the used setting names and the expected default values.
+
 ## Basic Syntax
 
 The syntax generally looks like this: