[css-display] list-item syntax does not allow <display-inside>

[Loirooriol]

CSS Display says this in Generating Marker Boxes: the list-item keyword:

If no <display-inside> value is specified, the principal box’s inner display type defaults to flow.

However, the syntax of <display-listitem> does not allow <display-inside>, only flow or flow-root:

<display-listitem> = list-item && <display-outside>? && [ flow | flow-root ]?

Yes, flow and flow-root are <display-inside> values, but this seems a coincidence. The spec should say that, in a <display-listitem> value, the flow and flow-root keywords (if any) behave like they do in <display-inside>.

[tabatkins]

Fixed to refer to "inner display type value" and "outer display type value", so I don't need to repeat the exact values already allowed in the grammar.