Fix #4747: BUR layout partly broken on iOS.

Move the BUR help text from the <textarea> placeholder attribute to a
<details> tag that embeds the [[help:bur_notice]] wiki page.

Also update some CSS for the <details> tag that was meant to only apply
to the user upgrades page and didn't look good here.

app/javascript/src/styles/specific/user_upgrades.scss

@@ -1,5 +1,9 @@
 div#c-user-upgrades {
   div#a-new {
+    summary {
+      padding: 1rem 0;
+    }
+
     table#feature-comparison {
       colgroup#basic {
         background-color: var(--user-upgrade-basic-background-color);