docs: add description to extendable eslint configs [ci skip] (#1862)

Author: st-sloth

packages/@vue/eslint-config-airbnb/README.md

@@ -1,3 +1,11 @@
 # @vue/eslint-config-airbnb
 
 > eslint-config-airbnb for vue-cli
+
+This config is specifically designed to be used by `vue-cli` setups
+and is not meant for outside use (it can be used but some adaptations
+on the user side might be needed - for details see the config file).
+
+A part of its design is that this config may implicitly depend on
+other parts of `vue-cli` setups, such as `eslint-plugin-vue` being
+extended in the same resulting config.

packages/@vue/eslint-config-prettier/README.md

@@ -1,3 +1,11 @@
 # @vue/eslint-config-prettier
 
 > eslint-config-prettier for vue-cli
+
+This config is specifically designed to be used by `vue-cli` setups
+and is not meant for outside use (it can be used but some adaptations
+on the user side might be needed - for details see the config file).
+
+A part of its design is that this config may implicitly depend on
+other parts of `vue-cli` setups, such as `eslint-plugin-vue` being
+extended in the same resulting config.

packages/@vue/eslint-config-standard/README.md

@@ -1,3 +1,11 @@
 # @vue/eslint-config-standard
 
 > eslint-config-standard for vue-cli
+
+This config is specifically designed to be used by `vue-cli` setups
+and is not meant for outside use (it can be used but some adaptations
+on the user side might be needed - for details see the config file).
+
+A part of its design is that this config may implicitly depend on
+other parts of `vue-cli` setups, such as `eslint-plugin-vue` being
+extended in the same resulting config.

packages/@vue/eslint-config-typescript/README.md

@@ -1,3 +1,11 @@
 # @vue/eslint-config-typescript
 
-> eslint-config-typescript for vue-cli
+> eslint-config-typescript for vue-cli
+
+This config is specifically designed to be used by `vue-cli` setups
+and is not meant for outside use (it can be used but some adaptations
+on the user side might be needed - for details see the config file).
+
+A part of its design is that this config may implicitly depend on
+other parts of `vue-cli` setups, such as `eslint-plugin-vue` being
+extended in the same resulting config.

packages/@vue/eslint-config-typescript/index.js

@@ -1,5 +1,9 @@
 module.exports = {
   plugins: ['typescript'],
+  // Prerequisite `eslint-plugin-vue`, being extended, sets
+  // root property `parser` to `'vue-eslint-parser'`, which, for code parsing,
+  // in turn delegates to the parser, specified in `parserOptions.parser`:
+  // https://github.com/vuejs/eslint-plugin-vue#what-is-the-use-the-latest-vue-eslint-parser-error
   parserOptions: {
     parser: require.resolve('typescript-eslint-parser')
   },