sp-core: Improve docs generated by `generate_feature_enabled_macro` (#4968)

Now: 

![image](https://github.com/paritytech/polkadot-sdk/assets/5718007/01545872-e21b-48b9-9913-4b940ff3f361)

Before:

![image](https://github.com/paritytech/polkadot-sdk/assets/5718007/09ae9fa4-eca8-43f7-a083-a4555e392610)


We sadly can not remove the `_macro_name`, but this way we at least show
the correct macro plus docs.