add a section to Configuration.md

Author: rchaser53

Configurations.md

@@ -2408,6 +2408,25 @@ pub enum Foo {}
 pub enum Foo {}
 ```
 
+## `inline_attribute_width` (tracking issue #3343):
+
+Write an item and its attributes on the same line if their combined width is below a threshold
+
+### Example
+
+Original Code (rustfmt will not change it with the default value of 0):
+
+#### `0`:
+```rust
+#[cfg(feature = "alloc")]
+use core::slice;
+```
+
+#### `50`:
+```rust
+#[cfg(feature = "alloc")] use core::slice;
+```
+
 ## `emit_mode`
 
 Internal option