Java config docs: default-value should be immutable

[aledsage]

No description provided.

[nakomis]

I think it would be worth changing the wording to make it clearer that you're not talking about the user setting the default to a new value, but instead modifying the underlying object, i.e. the risk isn't that they might say myConfigKey.setDefaultValue(newValue), but rather myListConfigKey.getDefaultValue().add(foo). Perhaps something like:

If supplying an object as a default value, it is important that the object is immutable. Otherwise, it risks users of the blueprint modifying the underlying object, which would affect blueprints that are subsequently deployed.

[drigodwin]

Thanks for the addition @aledsage, do you think this should replace @bostko's #171?

[geomacy]

LGTM, merging.