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/base/020_base.scss

@@ -112,13 +112,10 @@ table tfoot {
 }
 
 details {
-  border-bottom: 1px solid var(--details-border-color);
-
   summary {
     cursor: pointer;
     user-select: none;
     outline: none;
-    line-height: 2em;
   }
 }
 

app/javascript/src/styles/base/040_colors.css

@@ -255,7 +255,7 @@ html {
 
   --login-link-color: var(--red-5);
   --footer-border-color: var(--grey-1);
-  --details-border-color: var(--grey-2);
+  --divider-border-color: var(--grey-2);
 
   --jquery-ui-widget-content-background: var(--body-background-color);
   --jquery-ui-widget-content-text-color: var(--text-color);
@@ -446,7 +446,7 @@ body[data-current-user-theme="dark"] {
 
   --login-link-color: var(--red-4);
   --footer-border-color: var(--grey-7);
-  --details-border-color: var(--grey-7);
+  --divider-border-color: var(--grey-7);
 
   --jquery-ui-widget-content-text-color: var(--text-color);
   --jquery-ui-widget-content-background: var(--grey-8);

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; }

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);