The Remarks Section says

Equivalent to A = A + B.

But that is not true. += operator uses __iadd__ method, if it is defined.
If it's not defined, then it uses the __add__ method.

__iadd__ method does addition in place.
__add__ method adds the two objects and returns a new object.

In depth discussion can be found here : https://stackoverflow.com/a/2347423/13345708

The Remarks can be edited to say

Equivalent to A = A + B, if iadd method is not defined