Documentation for Enum support

[penguincarol]

With enum support, we need documentation. The docs are held in a very similar style to the postgres docu. Postgres scatters the documentation for some of the related functionality (e.g. drop type) a bit, which I am keeping more closely together.

Also, I am not sure to what extend we want to use running examples throughout the docu or not. I like them, but they arguably are more of a guide, than a documentation.

[mproebstl]

I think it would be good to also list the priority in the result and maybe explain it, e.g.,

select id, priority from tasks where priority >= 'major';

id | priority
----
1 | major
3 | critical
4 | major

ID 2 is filtered out because it has priority minor which was added before major and critical.

[penguincarol]

I see where you're coming from.
What do you think of using a much simlper example, like

select 'major'::importance>'minor'::importance;

instead? I feel like the slightly more complicated example currently presented in the doc is somewhat more intuitive, but you're right, that it is less self-explanatory.

[mproebstl]

I think both is fine. In general the one with the table is probably the more common use case, so I would go with this, but you can also do the other example or add both if you like.

[mproebstl]

thank you 🙏

[penguincarol]

you're welcome :D

[mproebstl]

I am unsure if we should maybe add some info about necessary permissions. Or should we only add this once the permissions are done? @ChrisWint

[penguincarol]

In a previous draft of this page I explained the permissions as such:

This change can only be performed by the current owner or a superuser. The user requires permissions to SET ROLE to the new owner aswell as USAGE on the schema of the enum.
The new owner requires CREATE on the previously mentioned schema.

However, I removed this before opening the PR as I do not describe the required permissions for the other statements either.