[feature] Better document conan/tools/env/environment.define_path

### What is your suggestion?

conan/tools/env/environment.define_path is mentioned in documentation https://docs.conan.io/2/reference/tools/env/environment.html?highlight=define_path but in the `Reference` section https://docs.conan.io/2/reference/tools/env/environment.html?highlight=prepend_path#reference is nowhere to be seen.

I guess it doesn't appear there becuse the method is not documented https://github.com/conan-io/conan/blob/release/2.0/conan/tools/env/environment.py#L220

### Have you read the CONTRIBUTING guide?

- [X] I've read the CONTRIBUTING guide