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/common/dtext.scss

@@ -131,6 +131,14 @@ div.prose {
   .spoiler {
     background: var(--dtext-spoiler-background-color);
   }
+
+  details {
+    margin-bottom: 1em;
+
+    summary {
+      margin-bottom: 1em;
+    }
+  }
 }
 
 // avoid empty gaps beneath dtext blocks in table rows.

app/javascript/src/styles/common/utilities.scss

@@ -42,6 +42,8 @@ $spacer: 0.25rem; /* 4px */
 .space-x-4 > * + * { margin-left: 4 * $spacer; }
 .space-y-4 > * + * { margin-top: 4 * $spacer; }
 
+.divide-y-1 > * + * { border-top: 1px solid var(--divider-border-color); }
+
 .align-top { vertical-align: top; }
 
 .flex-auto { flex: 1 1 auto; }