Clarify that DO_NOT_TRACK only disables telemetry for 1 or true

[kburym]

Fixes #45957

The GitHub CLI telemetry page implied any truthy value works for DO_NOT_TRACK. In internal/telemetry/telemetry.go (cli/cli at 0cf1092), DO_NOT_TRACK disables telemetry only when its trimmed, lowercased value is 1 or true. This adds that sentence next to the example.

[github-actions]

Please don't raise pull requests against an existing issue that another user has raised until it's been marked with a help wanted label. We need to properly assess all issues before we can accept community contributions, and determine whether it's something we need to fix internally, or how many articles are affected and what the scope of the fix will be. For now, we're going to close this pull request.

[github-actions]

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Source	Review	Production	What Changed
github-cli/github-cli/github-cli-telemetry.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18

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