Fix typo in generated documentation for entity! (#2176)

* entity: Fix typo in generated documentation

The same function documentation was used for `from_u32()` and `as_u32()`
while their behaviour is different

cranelift/entity/src/lib.rs

@@ -92,7 +92,7 @@ macro_rules! entity_impl {
         }
 
         impl $entity {
-            /// Return the underlying index value as a `u32`.
+            /// Create a new instance from a `u32`.
             #[allow(dead_code)]
             pub fn from_u32(x: u32) -> Self {
                 debug_assert!(x < $crate::__core::u32::MAX);