docs: add 'when not to use it' section in no-duplicate-case docs (#15563)

Fixes #15552

Author: mdjermanovic

docs/rules/no-duplicate-case.md

@@ -91,3 +91,18 @@ switch (a) {
         break;
 }
 ```
+
+## When Not To Use It
+
+In rare cases where identical test expressions in `case` clauses produce different values, which necessarily means that the expressions are causing and relying on side effects, you will have to disable this rule.
+
+```js
+switch (a) {
+    case i++:
+        foo();
+        break;
+    case i++: // eslint-disable-line no-duplicate-case
+        bar();
+        break;
+}
+```