Docs: explain config option merging (#7499)

* Update docs to explain config option merging

There was a lot of [confusion](#6361) generated by the merging behavior of rules inherited from base configs, so I thought I would clear that up here. There still may be [scenarios](#6144) which arise where options cannot be overridden from base configs, but we'll deal with those on a per-case basis.

* Use 4 spaces, not 2

* Use string notation, instead of numeric

* Fix level

docs/user-guide/configuring.md

@@ -566,8 +566,14 @@ ESLint extends configurations recursively so a base configuration can also have
 The `rules` property can do any of the following to extend (or override) the set of rules:
 
 * enable additional rules
-* override default options for rules from base configurations
-* disable rules from base configurations
+* change an inherited rule's severity without changing its options:
+    * Base config: `"eqeqeq": ["error", "allow-null"]`
+    * Derived config: `"eqeqeq": "warning"`
+    * Resulting actual config: `"eqeqeq": ["warning", "allow-null"]`
+* override options for rules from base configurations:
+    * Base config: `"quotes": ["error", "single", "avoid-escape"]`
+    * Derived config: `"quotes": ["error", "single"]`
+    * Resulting actual config: `"quotes": ["error", "single"]`
 
 ### Using `"eslint:recommended"`