Clarify JFROG_CLI_RELEASES_REPO env var documentation

[rickbrouwer]

• All tests have passed. If this feature is not already covered by the tests, new tests have been added.
• The pull request is targeting the master branch.
• The code has been validated to compile successfully by running go vet ./....
• The code has been formatted properly using go fmt ./....

---

The env var docs only stated that the remote should proxy https://releases.jfrog.io, without mentioning that the CLI appends 'artifactory/oss-release-local/' to the request URL. This caused confusion at our end configuring a remote pointing at a sub-path or a non-standard upstream. Spell out the exact URL that gets built and the root-proxy requirement.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[rickbrouwer]

I have read the CLA Document and I hereby sign the CLA

[rickbrouwer]

recheck