api: document preauthorized auth keys

Fixes #2120

Change-Id: If6a803680b544df1f70449c26fd0f5e15940226b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f