Document all `download_cache`'s properly

I would like to cache downloads of sources.
I look into documentation [documentation](https://docs.conan.io/2.0/reference/config_files/global_conf.html) and see that it mentions `core.download:download_cache` configuration and describes it as

> Define path to a file download cache

Are sources files? Yep. This must be a right fit. And it's not because this conf [controls path to a cache of internal files](https://github.com/conan-io/docs/issues/3276). It would be great to mention that in docs.

Then we have `tools.files.download:download_cache` configuration and the documentation says that it's a conf for

> caching the external/third_party files downloaded by recipes in download() or get() calls.

So I set it and get an error from conan

> ERROR: Unknown conf 'tools.files.download:download_cache'.

And... This conf doesn't exist. It turns out that this conf [was deleted](https://github.com/conan-io/conan/pull/13461). Changelogs doesn't mention that fact at all. This conf was replaced (as far as I understand) by `core.sources:download_cache` which is not documented at all.

Summing up

- `core.download:download_cache` conf documentation should say that it's for internal files
- `tools.files.download:download_cache` conf documentation should be removed
- `core.sources:download_cache` conf should be documented