docs: add note to user guide

Author: G-Rath

docs/user-guide/index.md

@@ -73,6 +73,8 @@ You can use the following configs by adding them to `eslint.config.js`.
 
 :::warning Reporting rules
 By default, all rules from **base** and **essential** categories report ESLint errors. Other rules - because they're not covering potential bugs in the application - report warnings. What does it mean? By default - nothing, but if you want - you can set up a threshold and break the build after a certain amount of warnings, instead of any. More information [here](https://eslint.org/docs/user-guide/command-line-interface#handling-warnings).
+
+Alternatively, you can set `process.env.VUE_ESLINT_ALWAYS_ERROR` to `true` in your configuration file to have `error` be used by all rules.
 :::
 
 #### Specifying Globals (`eslint.config.js`)