Document use of authenticated `client` and `django_user_model`

[shacker]

From my first reading of the docs, it appeared that pytest-django provided an authenticatable admin user (admin_client) or an anonymous standard user (client) but no way to authenticate a normal user. Only after reading the source code did I discover that client.login() was possible (which makes sense now that I grok that it wraps Django's test client, but wasn't clear at first).

So I started to document that, then realized the docs could use additional general cleanup in places, so am making a PR for a general helpers doc tweak.