100% doc coverage

@samphippen I wasn't sure what to do with the remaining any instance stuff, could you please have a look?

```
Undocumented Objects:

(in file: lib/rspec/mocks/any_instance/chain.rb)
RSpec::Mocks::AnyInstance
RSpec::Mocks::AnyInstance::Chain::Customizations
RSpec::Mocks::AnyInstance::Chain::Customizations.record
RSpec::Mocks::AnyInstance::Chain

(in file: lib/rspec/mocks/configuration.rb)
RSpec::Mocks::Configuration#yield_receiver_to_any_instance_implementation_blocks=
```

@myronmarston 

Fixes #585