Give WARNING paragraphs a splash of color

I'm not sure if this is necessary. I was playing around and this didn't
look terrible. Though it will probably turn x509.h into a sea of yellow
when it's ready to be rendered.

Change-Id: I34b26aad8a779a3fde761558d15b64c79159892a
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/64931
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

util/doc.css

diff --git a/util/doc.css b/util/doc.css
index f176f25..05d90be 100644
--- a/util/doc.css
+++ b/util/doc.css
@@ -56,6 +56,16 @@
   border-radius: 5px;
 }
 
+p.warning {
+  background-color: #fef5d3;
+  padding: 5px;
+  border-radius: 5px;
+}
+
+p.warning .first-word {
+  font-weight: bold;
+}
+
 .comment pre {
   margin-left: 2em;
 }

util/doc.go

diff --git a/util/doc.go b/util/doc.go
index 21d9dab..1de7064 100644
--- a/util/doc.go
+++ b/util/doc.go
@@ -641,7 +641,11 @@
 
 		switch block.Type {
 		case CommentParagraph:
-			b.WriteString("<p>")
+			if strings.HasPrefix(block.Paragraph, "WARNING:") {
+				b.WriteString("<p class=\"warning\">")
+			} else {
+				b.WriteString("<p>")
+			}
 			b.WriteString(string(markupParagraph(allDecls, block.Paragraph)))
 			b.WriteString("</p>")
 		case CommentOrderedListItem, CommentBulletListItem: