Docs: update no-unused-disable doc

Author: mysticatea

docs/rules/no-unused-disable.md

@@ -38,3 +38,29 @@ var foo = bar() //eslint-disable-line no-undef
 
 doSomething()
 ```
+
+## Known limitations
+
+This rule might not work fine if the rule wasn't loaded from [CLIEngine](https://eslint.org/docs/developer-guide/nodejs-api#cliengine) class because this rule is hacky a bit.
+
+Previously, this rule had depended on a private API of ESLint, but the private API was removed at ESLint 5.0.0 (see [eslint/eslint#10140](https://github.com/eslint/eslint/issues/10140) for details).
+So this rule is now using two public APIs as an alternative hack.
+
+- [require.cache](https://nodejs.org/api/modules.html#modules_require_cache)
+- [Linter.prototype.verify](https://eslint.org/docs/developer-guide/nodejs-api#linterverify)
+
+And it needs an assumption:
+
+- The rule was loaded from `CLIEngine` API.<br>
+  (I.e., the `eslint` module was loaded already, but the `Linter.prototype.verify` method hasn't been called yet.)
+
+### How it works
+
+When the rule is loaded, it searches `eslint` module from `require.cache` API and it overrides `Linter.prototype.verify` method.
+
+The overridden `Linter.prototype.verify` method does the following steps:
+
+1. If `config.rules["eslint-comments/no-unused-disable"]` is not enabled, it calls `super.verify` and returns the result as is.
+2. Otherwise, it calls `super.verify` with `reportUnusedDisableDirectives` option.
+3. It converts the result of `super.verify` as from `reportUnusedDisableDirectives` errors to `eslint-comments/no-unused-disable` errors.
+4. It returns the converted result.