Improve docs for Mapper::identity_map

src/structures/paging/mapper/mod.rs

@@ -351,6 +351,10 @@ pub trait Mapper<S: PageSize> {
 
  /// Maps the given frame to the virtual page with the same address.
  ///
+ /// This function might need additional physical frames to create new page
+ /// tables. These frames are allocated from the `frame_allocator` argument.
+ /// At most three frames are required.
+ ///
  /// ## Safety
  ///
  /// This is a convencience function that invokes [`map_to`] internally, so