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