Improve docs for citus_create_restore_point

## Communication goals (e.g. detailed howto vs orientation)

We have the main content written, but it should include more about the limitations, and inner workings.

### Good locations for content in docs structure

http://docs.citusdata.com/en/v7.5/develop/api_udf.html#citus-create-restore-point

## How does this work? (devs)

### Link to relevant commits and regression tests if applicable

https://github.com/citusdata/citus/pull/1452