[bug] core.net.http:proxies format is unclear

### Describe the bug

Conan version: 2.2.2

[Documentation](https://docs.conan.io/2/reference/config_files/global_conf.html) says `core.net.http:proxies: Dictionary containing the proxy configuration` without clarification the format of dictionary.

After some experiments I found that the key is a hostname and the value is port. So:
1. Please confirm that this is the expected format.
2. Update documentation to make this clear.
3. Ideally enforce the validation of the format of global.conf to ensure that it's always valid.