fix: nested {% comment %} tag broke the Pages build entirely

docs/_includes/head.html

@@ -14,11 +14,20 @@
   (meta tags, seo tag, stylesheet, feed_meta, conditional Google Analytics)
   so nothing else regresses; the JSON-LD block is the only addition.
 
-  This has to be a Liquid {% comment %} block, not an HTML <!-- --> comment:
-  Liquid tags are evaluated wherever they appear, even inside HTML comments,
-  and {% seo %} below renders its own "<!-- End Jekyll SEO tag -->" — whose
-  "-->" would close an HTML comment early and leak the rest of this text
-  (plus a duplicate <head>) straight into the rendered page.
+  Keep this a Liquid comment block, and never write Liquid tag syntax inside
+  it — not even as an example. Two separate outages came from ignoring that:
+
+  1. As an HTML comment, the seo tag named below still rendered (Liquid
+     evaluates tags anywhere, comments included). It emits its own HTML
+     end-comment marker, whose "--" + ">" closed this comment early and
+     leaked the remaining prose plus a duplicate head element into the page.
+  2. Rewritten as a Liquid comment, spelling out the opening comment tag in
+     prose opened a *nested* comment block, so the closing tag below matched
+     the inner one and the outer block was never closed. That's a hard build
+     failure: "Liquid syntax error: 'comment' tag was never closed".
+
+  Refer to tags by name only (seo tag, feed_meta, endcomment), never in
+  brace-percent form.
 {% endcomment %}
 <head>
   <meta charset="utf-8">