docs(@nestjs/swagger): jest troubleshooting info

Add instruction on how to clear the jest cache folder in case changes are not picked up

fixes: nestjs/swagger#1326

Author: mitsos1os

content/openapi/cli-plugin.md

@@ -197,3 +197,21 @@ With this in place, import AST transformer within your `jest` configuration file
   }
 }
 ```
+
+#### Troubleshooting `jest` (e2e tests)
+In any case that `jest` does not seem to pick up your configuration changes, it is quite possible that it has already **cached** the build result. In order for your changes _(`@nestjs/swagger/plugin` integration)_, to be taken into account, you need to clear its cache directory.
+
+In order to clear the cache directory of `jest`, run the following commands:
+```bash
+# Find out jest cache directory (usually /tmp/jest_rs)
+# by running command below in your NestJS project root
+$ npx jest --showConfig | grep cache
+# ex result:
+#   "cache": true,
+#   "cacheDirectory": "/tmp/jest_rs"
+
+# Remove or empty the found Jest cache directory
+$ rm -rf  <cacheDirectory value>
+# ex:
+# rm -rf /tmp/jest_rs  
+```