Clearly document shutting down other instances for index operations

This was originally discussed in #3965.

**Describe the feature:**
Many posts on our mailing list, SO and so on contain the recommendation to make sure that no management transaction is open when performing index operations to make sure that index operations aren't hindered by dangling open instances, but this isn't described clearly in the docs.

The [indexing docs](https://docs.janusgraph.org/schema/index-management/index-performance/#composite-index) currently mention that no other transaction should be open, but only in a code listing and also only really in a comment:

```java
graph.tx().rollback() //Never create new indexes while a transaction is active
```

The docs should make it more clear how important that is and also that other JanusGraph instances shouldn't be running in parallel as they could otherwise open new transactions after this has been executed.

**Describe a specific use case for the feature:**
Prevent users from running into a situation where an index isn't enabled only because some old management instance is still registered.

Read the indexing docs at the composite-index section, especially the existing graph.tx().rollback() example and its comment. Update the documentation so it clearly warns about open management transactions and other JanusGraph instances running in parallel. Done means the risk of an index remaining disabled because of an old management instance is explicit.

Written by the indexing model from the issue text.