Add docs for reassigning module vars. (#546)

Co-authored-by: Natalie Weizenbaum <nweiz@google.com>

source/documentation/at-rules/use.html.md.erb

@@ -378,6 +378,46 @@ another mixin to inject your styles.
   }
 <% end %>
 
+### Reassigning Variables
+
+After loading a module, you can reassign its variables.
+
+<% example(autogen_css: false) do %>
+  // _library.scss
+  $color: red;
+  ---
+  // _override.scss
+  @use 'library';
+  library.$color: blue;
+  ---
+  // style.scss
+  @use 'library';
+  @use 'override';
+  @debug library.$color;  //=> blue
+  ===
+  // _library.sass
+  $color: red
+  ---
+  // _override.sass
+  @use 'library'
+  library.$color: blue
+  ---
+  // style.sass
+  @use 'library'
+  @use 'override'
+  @debug library.$color  //=> blue
+<% end %>
+
+This even works if you import a module without a namespace using `as *`.
+Assigning to a variable name defined in that module will overwrite its value in
+that module.
+
+<% heads_up do %>
+  Built-in module variables (such as [`math.$pi`]) cannot be reassigned.
+
+  [`math.$pi`]: /documentation/modules/math#$pi
+<% end %>
+
 ## Finding the Module
 
 It wouldn't be any fun to write out absolute URLs for every stylesheet you load,