[css-mixins-1] Remove mentions of @nest (#10501)

css-mixins-1/Overview.bs

@@ -136,7 +136,7 @@ contains the same <<custom-property-name>> more than once,
 then the ''@function'' rule is invalid.
 
 The body of a ''@function'' rule accepts [=conditional group rules=],
-such as ''@media'', as well as the ''@nest'' rule.
+such as ''@media''.
 Additionally, it accepts the following descriptors:
 
  * The '@function/result' descriptor,
@@ -498,18 +498,6 @@ or acting as if nothing exists at that location otherwise.
  </pre>
 </div>
 
-
-Interaction with @nest {#at-nest}
-----------------------------
-
-''@nest'' rules are valid within ''@function'',
-with the additional restriction
-that only descriptors allowed within ''@function''
-are allowed within the ''@nest'' rule.
-
-A ''@nest'' rule within ''@function'' does nothing:
-it acts like its contents were present at its location.
-
 <!-- Big Text: cssom
 
  ███▌ ███▌ ███▌ ███▌ █ █
@@ -534,7 +522,7 @@ interface CSSFunctionRule : CSSGroupingRule { };
 While declarations may be specified directly within a ''@function'' rule,
 they are not represented as such in the CSSOM.
 Instead, consecutive segments of declarations
-appear as if wrapped in ''@nest'' rules.
+appear as if wrapped in {{CSSNestedDeclarations}} rules.
 
 Note: This also applies to the "leading" declarations in the ''@function'' rule,
  i.e those that do not follow another nested rule.
@@ -555,22 +543,22 @@ Note: This also applies to the "leading" declarations in the ''@function'' rule,
 
  <pre class='lang-css'>
  @function --bar() {
- @nest {
+ /* CSSNestedDeclarations { */
  --x: 42;
  result: var(--y);
- }
+ /* } */
  @media (width > 1000px) {
  /* ... */
  }
- @nest {
+ /* CSSNestedDeclarations { */
  --y: var(--x);
- }
+ /* } */
  }
  </pre>
 </div>
 
-Issue: Should we indeed use ''@nest'' for this purpose?
- The <code>style</code> attribute of the ''@nest'' rule
+Issue: Should we indeed use {{CSSNestedDeclarations}} for this purpose?
+ The <code>style</code> attribute of the {{CSSNestedDeclarations}} rule
  should probably not be a regular {{CSSStyleDeclaration}},
  since only custom properties
  and the '@function/result' descriptor