Write about best-practices for interface names in the same place they're defined

Write about best-practices for interface names in the same place they're defined

I'm deliberately using a hypothetical API from example.com, rather
than a real API, to avoid perpetuating these over-simplifications any
further than they've spread already:

- "namespaces start with org.freedesktop"
- "namespaces start with org"
- "interfaces are defined by their sole implementation"
- "services have one object implementing one interface"
- "interfaces always behave like classes"
- "interfaces are always noun phrases"
- "there is a freedesktop.org D-Bus API for screensavers"