Update concatenation docs to include relaxed and changes to rechunk d…

…efault behaviour

docs/user-guide/transformations/concatenation.md

@@ -21,7 +21,6 @@ Vertical concatenation fails when the dataframes do not have the same column nam
 
 For certain differences in dtypes, Polars can do a relaxed vertical concatenation where the differences in dtype are resolved by casting all columns with the same name but different dtypes to a *supertype*. For example, if column `'a'` in the first `DataFrame` is `Float32` but column `'a'` in the second `DataFrame` is `Int64`, then both columns are cast to their supertype `Float64` before concatenation. If the set of dtypes for a column do not have a supertype, the concatenation fails. The supertype mappings are defined internally in Polars.
 
-
 {{code_block('user-guide/transformations/concatenation','vertical_relaxed',['concat'])}}
 
 ```python exec="on" result="text" session="user-guide/transformations/concatenation"