[css-logical-1] Define 'inset properties' here because it's not defined anywhere else yet. :/

Author: fantasai

css-logical-1/Overview.bs

@@ -442,6 +442,11 @@ the 'margin-block-start', 'margin-block-end', 'margin-inline-start', 'margin-inl
 Flow-relative Offsets:
 the 'inset-block-start', 'inset-block-end', 'inset-inline-start', 'inset-inline-end' properties and 'inset-block', 'inset-inline', and 'inset' shorthands</h3>
 
+  The 'top', 'left', 'bottom', 'right' physical properites,
+  their 'inset-block-start', 'inset-block-end', 'inset-inline-start', 'inset-inline-end' flow-relative correspondants,
+  and the 'inset-block', 'inset-inline', and 'inset' shorthands,
+  are colectively known as the <dfn export>inset properties</dfn>.
+
   <pre class="propdef">
   Name: inset-block-start, inset-block-end, inset-inline-start, inset-inline-end
   Value: <<'top'>>