add feature doc

docs/user/features.md

@@ -406,6 +406,35 @@ fn f() {
 }
 ```
 
+- Move if condition to match arm guard
+```rust
+// before:
+fn f() {
+    let mut t = 'a';
+    let chars = "abcd";
+    match t {
+        '\r' => if chars.clone().next().is_some() {
+            t = 'e';<|>
+            false
+        },
+        _ => true
+    }
+}
+
+// after:
+fn f() {
+    let mut t = 'a';
+    let chars = "abcd";
+    match t {
+        '\r' <|>if chars.clone().next().is_some() => {
+            t = 'e';
+            false
+        },
+        _ => true
+    }
+}
+```
+
 ### Magic Completions
 
 In addition to usual reference completion, rust-analyzer provides some ✨magic✨