Update guidance concerning mixing arrays with Python scalars to handle complex

[kgryte]

This PR

• updates the guidance concerning mixing arrays with Python scalars. The current specification accommodates bool and real-valued data types. This PR updates the guidance to accommodate mixing a complex Python scalar with arrays. Guidance is intentionally limited to when a scalar is complex and an array has a complex floating-point array data type.
• does not specify what happens when a scalar is complex and an array has a real-valued array data type. Behavior is left implementation-specific (e.g., promotion, exception, etc). Given that current guidance requires converting a scalar to the array data type, to accommodate complex scalars and real-valued arrays, we'd need to specify casting rules for complex to real, which is something the specification has thus far avoided (e.g., astype, real, imag, conj, et al). This PR aims to be consistent with those prior decisions.

[rgommers]

This looks logical and straightforward to me, so I'll merge it. Thanks @kgryte