Update issue template (#15886)

Co-authored-by: Alexander Kachkaev <alexander@kachkaev.ru>

.github/ISSUE_TEMPLATE/formatting.md

@@ -46,4 +46,12 @@ Don't fill the form below manually! Let a program create a report for you:
 // code snippet
 ```
 
-**Expected behavior:**
+**Expected output:**
+
+```jsx
+// code snippet
+```
+
+**Why?**
+
+<!-- short explanation of expected output -->

website/playground/markdown.js

@@ -27,7 +27,10 @@ function formatMarkdown({
     ...(isIdempotent
       ? []
       : ["", "**Second Output:**", codeBlock(output2, syntax)]),
-    ...(full ? ["", "**Expected behavior:**", ""] : []),
+    ...(full ? ["", "**Expected output:**", codeBlock("", syntax)] : []),
+    ...(full
+      ? ["", "**Why?**", "", "<!-- short explanation of expected output -->"]
+      : []),
   ]
     .filter((part) => part !== null)
     .join("\n");