docs: clarify the expected results for in-place operations for immutable array libraries

[kgryte]

This PR

• closes Clarification for non-portable behavior for in-place Python operations #828 by indicating that, for libraries which do not implement in-place operators, the equivalence between x1 += x2 and x1 = x1 + x2 only holds when the result of type promotion between x1 and x2 is equal to the dtype of x1. When this does not hold, behavior is unspecified and thus implementation-defined.
• fixes guidance regarding allowed dtypes for the matmul operator, which was an oversight when adding complex dtype support in the v2023 revision of the standard.

cc @jakevdp @rgommers @oleksandr-pavlyk @seberg