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/2850Reviewed-by: Russ Cox <rsc@golang.org>

test/cmplxdivide.c

@@ -2,6 +2,17 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// This C program generates the file cmplxdivide1.go. It uses the
+// output of the operations by C99 as the reference to check
+// the implementation of complex numbers in Go.
+// The generated file, cmplxdivide1.go, is compiled along
+// with the driver cmplxdivide.go (the names are confusing
+// and unimaginative) to run the actual test. This is done by
+// the usual test runner.
+//
+// The file cmplxdivide1.go is checked in to the repository, but
+// if it needs to be regenerated, compile and run this C program
+// like this:
 //	gcc '-std=c99' cmplxdivide.c && a.out >cmplxdivide1.go
 
 #include <complex.h>

test/cmplxdivide.go

@@ -5,6 +5,7 @@
 // license that can be found in the LICENSE file.
 
 // Driver for complex division table defined in cmplxdivide1.go
+// For details, see the comment at the top of in cmplxdivide.c.
 
 package main