document plugin resolution as subcommand for builtin commands

content/en/docs/reference/kubectl/kubectl.md

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+<td colspan="2">KUBECTL_ENABLE_CMD_SHADOW</td>
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+<td></td><td style="line-height: 130%; word-wrap: break-word;">When set to true, external plugins can be used as subcommands for builtin commands if subcommand does not exist. In alpha stage, this feature can only be used for create command(e.g. kubectl create networkpolicy). 
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