A lot more documentation.

docs/clone_interface.md

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+# `clone_interface` Command
+
+## Name
+
+`clone_interface`
+
+## Synopsis
+
+```
+clone_interface [<nic>]...
+```
+
+## Description
+
+Adds the specified NICs to the `cloned_interfaces` variable.  This is a
+friendly wrapper for `cloned_interfaces="${cloned_interfaces} $1"` in essence.
+This command can be safely mixed with any raw variable setting of
+`cloned_interfaces`, as long as care is taken in the raw variable setting
+code not to overwrite the variable, but instead append to it.
+
+## See Also
+
+ - [`add_vlan`](add_vlan.md)
+ - [`add_wireguard_interface`](add_wireguard_interface.md)