docs: add leading pipe config details

Author: calebcartwright

Configurations.md

@@ -2001,6 +2001,55 @@ fn main() {
 }
 ```
 
+## `strip_leading_match_arm_pipes`
+
+Controls whether to remove leading pipes from match arms
+
+- **Default value**: `true`
+- **Possible values**: `true`, `false`
+- **Stable**: Yes
+
+#### `true`:
+```rust
+// Leading pipes are from this:
+// fn foo() {
+//     match foo {
+//         | "foo" | "bar" => {}
+//         | "baz"
+//         | "something relatively long"
+//         | "something really really really realllllllllllllly long" => println!("x"),
+//         | "qux" => println!("y"),
+//         _ => {}
+//     }
+// }
+
+// Are removed:
+fn foo() {
+    match foo {
+        "foo" | "bar" => {}
+        "baz"
+        | "something relatively long"
+        | "something really really really realllllllllllllly long" => println!("x"),
+        "qux" => println!("y"),
+        _ => {}
+    }
+}
+```
+
+#### `false`:
+```rust
+fn foo() {
+    match foo {
+        | "foo" | "bar" => {}
+        | "baz"
+        | "something relatively long"
+        | "something really really really realllllllllllllly long" => println!("x"),
+        | "qux" => println!("y"),
+        _ => {}
+    }
+}
+```
+
 ## `struct_field_align_threshold`
 
 The maximum diff of width between struct fields to be aligned with each other.