Document why skip-prune is not defailt

[jamietanna]

Via:

         Probably not for this PR, but it would make sense to not prune code that is used in dependent APIs by default, instead of having to set this explicitly.

Originally posted by @markuswustenberg in #1389 (comment)

[jamietanna]

This is called out in https://github.com/deepmap/oapi-codegen?tab=readme-ov-file#key-design-decisions - do we want any more than this?

[markuswustenberg]

The issue is about not pruning code that is used in other, dependent APIs, where those APIs are known when generating, I think? In which case it would make sense to not prune them by default, because they're used in the dependent APIs, and it otherwise leads to errors in the generated code.

TLDR: if an option is not optional but mandatory for the generated code to work, why is it an option?