document default values for options

[milyin]

Describe the release item

When creating entity it's not clear which settings it will have by default, if the setting is simple type. For complex types, implementing default() method it's more clear. E.g. for allowed_origin method we can look into definition of Locality type and ensure, that default is Locality::Any But for example for QueryableBuilder it's unknown is completeness option true or false by default without deep dive into sources.

This should be explicit. The rule is: if option accepts value of base type or if type is not base, but default value of this option is not the default value for the type, this default option value should be documented.

Maybe define constants for these values in the API, but this to be decided.

[milyin]

Mostly duplication of https://github.com/eclipse-zenoh/zenoh/issues/732, fixed in https://github.com/eclipse-zenoh/zenoh/pull/729