Document mixing real framework services with Moq AutoMock (#49)

[tillig]

Companion to autofac/Autofac.Extras.Moq#61, addressing autofac/Autofac.Extras.Moq#49.

Adds a "Mixing Real Framework Services with Auto-Mocking" section to the Moq integration docs (docs/integration/moq.rst).

It explains:

• AutoMock injects a mock for any unresolved service, including the elements of an otherwise-empty collection dependency (e.g. IEnumerable<ILoggerProvider>).
• A loose mock element returns null for un-setup members, which can make a consuming framework throw — frequently a NullReferenceException deep inside the framework (the EF Core + logging case from the issue).
• A note that the failure can appear intermittently (e.g. only in a certain test order) because frameworks like EF Core cache internal state that masks the problem.
• The workaround: register the concrete services the framework consumes (e.g. a real ILoggerFactory), or keep real framework wiring out of the auto-mock container.