[css-mixins-1] Refer properly to 'nested declarations rule' rather than an actual nested style rule.

Author: tabatkins

css-mixins-1/Overview.bs

@@ -845,7 +845,7 @@ making them reusable and customizable with arguments.
 
 	<pre highlight=css>
 		h1 {
-			& {
+			@nest {
 				@env --from: pink;
 				@env --to: powderblue;
 				@env --angle: to bottom right;
@@ -861,6 +861,9 @@ making them reusable and customizable with arguments.
 			}
 		}
 	</pre>
+
+	(Where <css>@nest</css> is a fictitious rule
+	representing a [=nested declarations rule=].)
 </div>
 
 Issue: The entire ''@mixin'' feature is experimental and under active development,
@@ -914,9 +917,9 @@ then the ''@mixin'' rule is invalid.
 <h4 id=mixin-body dfn lt="mixin body" export>
 The Mixin Body</h4>
 
-The body of a ''@mixin'' rule acts as a [=nested style rule=],
+The body of a ''@mixin'' rule acts as a [=nested declarations rule=],
 and accepts the same properties and rules
-that a normal [=nested style rule=] would.
+that a normal [=nested declarations rule=] would.
 
 In particular, further [=mixins=] can be invoked
 (via the ''@apply'' rule)
@@ -936,10 +939,10 @@ or those defined at the <em>call site</em>
 (a style rule, or another [=mixin=]).
 
 In that list, earlier things "win" over later things of the same name,
-exactly as if the [=mixin body=] was a [=nested style rule=]
+exactly as if the [=mixin body=] was a [=nested declarations rule=]
 placed at its call site.
-Specifically, it desugars to <em>two</em> [=nested style rules=],
-to correctly reproduce the argument scope
+Specifically, it desugars to <em>two</em> [=nested declarations rules=],
+to correctly reproduce the argument scope.
 
 <div class=example>
 	For example, the following mixin use:
@@ -960,15 +963,18 @@ to correctly reproduce the argument scope
 	<xmp highlight=css>
 	p.nested {
 		@env --color1: red;
-		& {
+		@nest {
 			@env --color2: green;
-			& {
+			@nest {
 				@env --color3: blue;
 				background: linear-gradient(env(--color1), env(--color2), env(--color3));
 			}
 		}
 	}
 	</xmp>
+
+	(Where <css>@nest</css> here is a fictitous rule
+	representing a [=nested declarations rule=].)
 </div>
 
 <div class=example>
@@ -1016,7 +1022,7 @@ Using Mixins {#using-mixins}
 
 The result of a [=mixin=] application
 is substituted into the body of another [=style rule=]
-as a [=nested style rule=]
+as a [=nested declarations rule=]
 via the ''@apply'' rule.
 
 <h3 id=apply-rule>