Add how to install watch fixtures yourself to the docs

When setting `grafserv: { watch: true }` and omitting `makePgService`'s `superUserConnectionString`, Postgraphile errors with the following text:

```
Installing watch fixtures requires superuser privileges; have you correctly configured a 'superuserConnectionString'?
You may also opt to configure 'installWatchFixtures: false' and install them yourself.
```

(Great error message, btw! Clear, helpful errors are an underrated part of UX 🙂)

When using Postgraphile myself, I want to install watch fixtures as a part of my db setup so that the server running postgraphile does not need access to super user db creds. I went to check the docs on how to do this, and noticed it wasn't in the docs anywhere. We should add how and why to do this to the docs.

I found the queries here: https://github.com/graphile/crystal/blob/36c41e80bfe312f584af42545688fb78d3adaf3f/graphile-build/graphile-build-pg/src/watchFixtures.ts

A quick 👍 to make sure we're aligned, and I'm happy to make the PR for this addition myself.