Slight visual improvements to warning boxes in the docs

src/librustdoc/html/static/rustdoc.css

@@ -858,6 +858,15 @@ body.blur > :not(#help) {
 	font-size: 1.5em;
 }
 
+/* Black one-pixel outline around emoji shapes */
+.emoji {
+	text-shadow:
+		1px 0 0 black,
+		-1px 0 0 black,
+		0  1px 0 black,
+		0 -1px 0 black;
+}
+
 .module-item .stab {
 	border-radius: 3px;
 	display: inline-block;