[Sass] Document !guarded.

doc-src/SASS_CHANGELOG.md

@@ -20,6 +20,12 @@
   it's possible that some properties will break.
   If this happens, please report it to [the Sass mailing list](http://groups.google.com/group/haml).
 
+* In addition, setting the default value of variables
+  with `||=` is now deprecated
+  and will be removed in Sass 3.2.
+  Instead, add `!default` to the end of the value.
+  See also [this changelog entry](#3-0-0-default-flag)
+
 * The `!` prefix for variables is deprecated,
   and will be removed in Sass 3.2.
   Use `$` as a prefix instead.
@@ -130,6 +136,17 @@ is compiled to:
       height: 250px;
       margin-left: 9px; }
 
+##### Variable Defaults
+
+Since `=` is no longer used for variable assignment,
+assigning defaults to variables with `||=` no longer makes sense.
+Instead, the `!default` flag
+should be added to the end of the variable value.
+This syntax is meant to be similar to CSS's `!important` flag.
+For example:
+
+    $var: 12px !default
+
 #### Variable Prefix Character
 {#3-0-0-dollar-prefix}
 

doc-src/SASS_REFERENCE.md

@@ -957,28 +957,29 @@ is compiled to:
     p.foo {
       border-color: blue; }
 
-### Optional Assignment: `||:`
+### Variable Defaults: `!default`
 
 You can assign to variables if they aren't already assigned
-using the `||:` assignment operator. This means that if the
-variable has already been assigned to, it won't be re-assigned,
+by adding the `!default` flag to the end of the value.
+This means that if the variable has already been assigned to,
+it won't be re-assigned,
 but if it doesn't have a value yet, it will be given one.
 
 For example:
 
 {.sass-ex}
     $content: "First content"
-    $content ||: "Second content?"
-    $new_content ||: "First time reference"
+    $content: "Second content?" !default
+    $new_content: "First time reference" !default
 
     #main
       content: $content
       new-content: $new_content
 
 {.scss-ex}
     $content: "First content";
-    $content ||: "Second content?";
-    $new_content ||: "First time reference";
+    $content: "Second content?" !default;
+    $new_content: "First time reference" !default;
 
     #main {
       content: $content;