Add a comment for escape-svg function

Closes #30835 by leaving a comment in the source that the escape-svg() function must have quotes around data URIs.

(cherry picked from commit 849fea5a1e)

scss/_functions.scss

@@ -51,6 +51,9 @@
 }
 
 // See https://codepen.io/kevinweber/pen/dXWoRw
+//
+// Requires the use of quotes around data URIs.
+
 @function escape-svg($string) {
   @if str-index($string, "data:image/svg+xml") {
     @each $char, $encoded in $escaped-characters {