derp: document the RunWatchConnectionLoop callback gotchas

Updates #13566

Change-Id: I497b5adc57f8b1b97dbc3f74c0dc67140caad436
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f