docs: improve doctests for complex number instances in array/complex128

PR-URL: #10718
Ref: #8641
Co-authored-by: Athan Reines <kgryte@gmail.com>
Reviewed-by: Athan Reines <kgryte@gmail.com> 
Co-authored-by: stdlib-bot <noreply@stdlib.io>

Author: 3 people