Paths to Django objects should be easier to find in the documentation

[Daniele Procida]

Our documentation doesn't always make it obvious where the objects it's talking about are to be found and imported from.

We do it in various different way. Sometimes:

• it's shown in an import in a code example
• an object's full dotted path is shown as part of its name
• it says explicitly at the start of a section where the objects mentioned in it will be found
• we don't say it at all

I am not sure what would be best, or whether one way could be used explicitly throughout.

[Tim Graham]

Duplicate of #15396