cmd/tailscale: clarify which prefless flags don't need revert protection

Fixes #3482

Change-Id: Icb51476b0d78d3758cb04df3b565e372b8289a46
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
(cherry picked from commit a8f60cf6e8)

cmd/tailscale/cli/up.go

@@ -46,8 +46,10 @@ down").
 
 If flags are specified, the flags must be the complete set of desired
 settings. An error is returned if any setting would be changed as a
-result of an unspecified flag's default value, unless the --reset
-flag is also used.
+result of an unspecified flag's default value, unless the --reset flag
+is also used. (The flags --authkey, --force-reauth, and --qr are not
+considered settings that need to be re-specified when modifying
+settings.)
 `),
 	FlagSet: upFlagSet,
 	Exec:    runUp,