citus_columnar as a separate extension and shared_preload_libraries

Hi!
citus_columnar is enabled as a separate extension since v11.1, but it is still not documented (#7189). [Release notes](https://www.citusdata.com/updates/v11-1/) say

> make sure that citus_columnar is in the shared_preload_libraries if you want to use only columnar features of Citus

Is it really needed for the correct work of citus_columnar? If so, then why when trying to
`CREATE EXTENSION citus_columnar`
without citus_columnar in the shared_preload_libraries there is no error? When doing the same for citus user gets
```
ERROR: Citus can only be loaded via shared_preload_libraries
HINT: Add citus to shared_preload_libraries configuration variable in postgresql.conf in master and workers. Note that citus should be at the beginning of shared_preload_libraries.
```

The same error for citus_columnar would be really helpful to prevent any problems that could occur when using citus_columnar as a separate extension without listing it in the shared_preload_libraries. It's especially important since no documentation is available and users can easily miss the necessity to specify shared_preload_libraries.