[css-mixins-1] Let 'inherit' work like 'inherit()' (w3c#12387)

With @function, it becomes possible to have a "type mismatch"
between an "outer" custom property and an "inner" custom property:

  @Property --x {
    syntax: "auto | red";
    /* ... */
  }

  @function --f(--x <color>: inherit) {
    result: var(--x);
  }

  div {
    --x: red;
    --f(); /* => ? */
  }

We have to address this somehow, and the obvious way seems to be
making 'inherit' behave like "a var(--x) that resolves in the parent
stack frame". This is essentially what inherit() does, which already
exists in the spec.

Author: andruud

css-mixins-1/Overview.bs

@@ -633,8 +633,29 @@ with its [=function parameters=] overriding "inherited" custom properties of the
 			is the [=guaranteed-invalid value=].
 
 		* On custom properties,
-			the [=CSS-wide keywords=] ''initial'' and ''inherit'' have their usual effect;
-			all other [=CSS-wide keywords=] resolve to the [=guaranteed-invalid value=].
+			the [=CSS-wide keywords=] have the following effects:
+
+			<dl>
+				<dt>''initial''</dt>
+				<dd>
+					Resolves to the initial value of the custom property
+					within |registrations|.
+				</dd>
+
+				<dt>''inherit''</dt>
+				<dd>
+					Resolves like an ''inherit()'' function
+					with the custom property name as its one and only argument.
+
+					Note: This ensures that a [=function parameter=] defaulted to ''inherit''
+						is reinterpreted using the local [=parameter type=].
+				</dd>
+
+				<dt>any other [=CSS-wide keyword=]</dt>
+				<dd>
+					Resolves to the [=guaranteed-invalid value=].
+				</dd>
+			</dl>
 
 			Note: ''initial'' references the [=custom property registration=]
 			created from the [=function parameters=],