test: comment the behavior and use of cmplxdivide*

The various files are confusingly named and their operation
not easy to see. Add a comment to cmplxdivide.c, one of the few
C files that will endure in the repository, to explain how to build
and run the test.

Change-Id: I1fd5c564a14217e1b9815b09bc24cc43c54c096f
Reviewed-on: https://go-review.googlesource.com/2850


Reviewed-by: Russ Cox <rsc@golang.org>