docs: cite Glimmer VM source verifying autofocus={{false}} omits attribute

The G3 exemption's load-bearing premise (Glimmer renders no attribute
when given a mustache-literal false) was asserted in the PR body but
unverified in the rule's own documentation. Verified against Glimmer
VM's attribute-normalization path:

  glimmer-vm/packages/@glimmer/runtime/lib/vm/attributes/dynamic.ts

`normalizeValue` returns null for false/undefined/null; then
`SimpleDynamicAttribute.update()` calls element.removeAttribute(name)
when the value is null. So autofocus={{false}} genuinely omits the
attribute from the rendered DOM — not autofocus="false".

Inlined the citation into the rule's JSDoc so future readers don't
have to trace it.

Author: johanrd

lib/rules/template-no-autofocus-attribute.js

@@ -13,6 +13,13 @@
  * syntax — Glimmer authors writing `autofocus={{false}}` are expressing
  * intent to omit the attribute conditionally. Treat that narrow literal
  * case as opt-out (the rendered HTML will have no autofocus attr).
+ *
+ * Verified against Glimmer VM's attribute-normalization source:
+ * glimmer-vm/packages/@glimmer/runtime/lib/vm/attributes/dynamic.ts —
+ * `normalizeValue` returns `null` for `false | undefined | null`, and
+ * `SimpleDynamicAttribute.update()` calls `element.removeAttribute(name)`
+ * when the normalized value is null. So `autofocus={{false}}` renders
+ * with the attribute entirely absent from the DOM.
  */
 function isMustacheBooleanFalse(value) {
   if (value?.type !== 'GlimmerMustacheStatement') {