Add some missing comments to describe what the inlined SVG is

src/librustdoc/html/static/css/rustdoc.css

@@ -1133,6 +1133,7 @@ so that we can apply CSS-filters to change the arrow color in themes */
 .setting-check input:checked {
 	background-color: var(--settings-input-color);
 	border-width: 1px;
+	/* cross-mark image in the settings checkboxes */
 	content: url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 40 40">\
 		<path d="M7,25L17,32L33,12" fill="none" stroke="black" stroke-width="5"/>\
 		<path d="M7,23L17,30L33,10" fill="none" stroke="white" stroke-width="5"/></svg>');
@@ -1627,6 +1628,7 @@ a.tooltip:hover::after {
 }
 
 #sidebar-button > a:before {
+	/* sidebar resizer image */
 	content: url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 22 22" \
 		fill="none" stroke="black">\
 		<rect x="1" y="1" width="20" height="20" ry="1.5" stroke-width="1.5"/>\
@@ -1855,6 +1857,7 @@ However, it's not needed with smaller screen width because the doc/code block is
 /* sidebar button opens modal
 	use hamburger button */
 .src #sidebar-button > a:before, .sidebar-menu-toggle:before {
+	/* hamburger button image */
 	content: url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" \
 		viewBox="0 0 22 22" fill="none" stroke="black">\
 		<path d="M3,5h16M3,11h16M3,17h16" stroke-width="2.75"/></svg>');
@@ -1868,6 +1871,7 @@ However, it's not needed with smaller screen width because the doc/code block is
 
 /* src sidebar button opens a folder view */
 .src #sidebar-button > a:before {
+	/* folder image */
 	content: url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" \
 		viewBox="0 0 22 22" fill="none" stroke="black">\
 		<path d="M16,9v-4h-6v-1l-2,-2h-4l-2,2v16h13L21,9h-15L2,19" stroke-width="1.25"/>\