proxy env variable doc

[anaxceron]

Describe your pull request here: Documenting how to use a proxy w/ Zowe CLI. Addressing Issue 3731.

List the file(s) included in this PR: docs\user-guide\cli-configuringcli-ev.md

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[sbohnert]

Looks good. One minor comment.

[adam-wolfe]

LGTM, thanks @anaxceron