[dcl.attr.fallthrough] "fallthrough statement" should be formatted as a definition

[eisenwave]

draft/source/declarations.tex

Lines 9784 to 9785 in 245d385

	\indextext{statement!fallthrough}
	such a statement is a fallthrough statement.

I don't know why, but we don't italicize the term and turn this into a proper definition, despite "fallthrough statement" clearly being defined here. It's also referenced in a number of places, including in the proposed resolution of CWG3045.