Update 'Purpose and Scope' for complex dtype support

[lucascolley]

'Purpose and Scope' still states that complex dtypes will be included in a future version.

4. Standardization of these dtypes is out of scope: bfloat16, complex, extended
   precision floating point, datetime, string, object and void dtypes.

Rationale: these dtypes aren't uniformly supported, and their inclusion at
this point in time could put a significant implementation burden on
libraries. It is expected that some of these dtypes - in particular
bfloat16, complex64, and complex128 - will be included in a future
version of the standard.