Clarify JFROG_CLI_RELEASES_REPO env var documentation

[rickbrouwer]

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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]

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[rickbrouwer]

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

[rickbrouwer]

recheck

[rickbrouwer]

@attiasas

Do you happen to have an opinion on the PR?
I once offered to improve some of the documentation.
But if there is no need for it, it can be closed.