Better Documentation of Tables With Two Foreign Keys to the Same Table
### Feature description

https://www.graphile.org/postgraphile/relations/ lists the most basic relations, including a basic many-to-many example, which is great if you have a "users" table, and an "employees" table with a foreign key to "users".

But what if you also have _another_ column in "employees" that _also_ is a foreign key to "users" (eg. a "supervisor" column)?  Then you get:

> Error: A naming conflict has occurred - two entities have tried to define the same key 'employees'.
> 
>   The first entity was:
>     ...
>   The second entity was:
>     ...

along with a bunch of text about using smart comments and such.

But surely it's possible to have two columns with the same foreign key target without smart comments?  And surely this is worth documenting?

### Motivating example

I gave a basic one (two "employee" and "supervisor" foreign key constraints to a "users" table), but there are LOTS of reasons to have two foreign keys in the same table.

### Breaking changes

Nope, just documentation.

### Supporting development

I [tick all that apply]:

Am happy to help, but since I don't know what the solution is or how to work on Postgraphile documentation my ability to do so is limited.
<!-- To tick a checkbox, change it from `[ ]` to `[x]` -->

- [ ] am interested in building this feature myself
- [ ] am interested in collaborating on building this feature
- [ ] am willing to help testing this feature before it's released
- [ ] am willing to write a test-driven test suite for this feature (before it exists)
- [ ] am a [Graphile sponsor](https://www.graphile.org/sponsor/) ❤️
- [ ] have an active [support or consultancy contract](https://www.graphile.org/support/) with Graphile
