Update type promotion rules for complex dtypes

- Add a table with promotion rules
- Update type promotion lattice diagram

Closes data-apisgh-477
Closes data-apisgh-478

Author: rgommers

spec/API_specification/type_promotion.rst

@@ -102,6 +102,25 @@ where
 -   **f4**: single-precision (32-bit) floating-point number (i.e., ``float32``)
 -   **f8**: double-precision (64-bit) floating-point number (i.e., ``float64``)
 
+Complex floating-point type promotion table
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
++---------+-----+-----+
+|         | c8  | c16 |
++=========+=====+=====+
+| **c8**  | c8  | c16 |
++---------+-----+-----+
+| **c16** | c16 | c16 |
++---------+-----+-----+
+
+where
+
+-   **c8**: single-precision complex floating-point number (i.e., ``complex64``)
+    composed of two single-precision (32-bit) floating-point numbers
+-   **c16**: double-precision complex floating-point number (i.e., ``complex128``)
+    composed of two double-precision (64-bit) floating-point numbers
+
+
 Notes
 ~~~~~