[css-anchor-position-1] Re-create anchor() function definition (w3c#9615)

The definition was inadvertently dropped in:
w3c@471d90f

Without the definition, Bikeshed links to the definition in the TR version of
the spec, but that definition will disappear next time the spec is updated.

Author: tidoust

css-anchor-position-1/Overview.bs

@@ -565,7 +565,7 @@ The ''anchor()'' Function {#anchor-pos}
 ---------------------------------------------------
 
 An [=absolutely-positioned=] element
-can use the ''anchor()'' function
+can use the <dfn>anchor()</dfn> function
 as a value in its [=inset properties=]
 to refer to the position of one or more [=anchor elements=].
 The ''anchor()'' function resolves to a <<length>>.