cmd/tailscale: fix, test some recent doc inconsistencies

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3dabea0fc2 added some docs with inconsistent usage docs.
This fixes them, and adds a test.

It also adds some other tests and fixes other verb tense
inconsistencies.

Updates tailscale/corp#25278

Change-Id: I94c2a8940791bddd7c35c1c3d5fb791a317370c2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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