The above assumes the use of the new JavaScript NGRE Rules (the ONE branch currently on github) which also assumes an upgraded openHAB version. If you are seeing REST API documentation under MISC, you are still running OH 2.4. I cannot guarantee that the v1.2 stuff will work on that old of an OH version. In particular there are potentially some changes in the MQTT Things that require a later version of OH.
See Interoperability with other OpenHAB instance for the steps necessary to do an in place upgrade from v1.1 to the WIP v1.2 version which includes the rewritten Rules and configurations and some new features.
If you stay on v1.1, which is reasonable, than this thread doesn’t apply to you at all. The Rules for v1.1 are written in default.rules so modifying them is a completely different procedure.
If you just want to access the REST API Docs, than indeed install if from the MISC tab instead of the UIs tab. But pretty much everything else in the OP will not apply to you.