SONAR-16596 Update "Security Engine Custom Configuration" doc page with common configuration

server/sonar-docs/src/pages/analysis/security_configuration.md

@@ -180,7 +180,7 @@ You need to add your custom configurations to SonarQube using a JSON file. You c
 See the following section for more information on formatting your JSON file.
 
 ### Configuration file format
-Your JSON file should include the rule you're adding a custom element to, the element you are customizing, and the `methodId` for each element. Each language needs a separate JSON file but can contain multiple rules. Click your language below to expand an example of a JSON file to help you understand the expected format.
+Your JSON file should include the rule you're adding a custom element to, the element you are customizing, and the `methodId` for each element. Each language needs a separate JSON file but can contain multiple rules. You may use the special rule key `common` to apply the given configuration to all the rules. Click your language below to expand an example of a JSON file to help you understand the expected format.
 
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 | ## Java JSON file example