cmd/cgo: document CC_FOR_TARGET and CXX_FOR_TARGET

Update #4714

LGTM=iant, minux.ma, rsc
R=rsc, iant, r, minux.ma
CC=golang-codereviews
https://golang.org/cl/100390043

doc/go1.3.html

@@ -226,6 +226,13 @@ supports a new <code>-i</code> option to install dependencies
 of the specified target, but not the target itself.
 </p>
 
+<p>
+Cross compiling with <a href="/cmd/cgo/"><code>cgo</code></a> enabled
+is now supported. The CC_FOR_TARGET and CXX_FOR_TARGET environment
+variables are used when running all.bash to specify the cross compilers
+for C and C++ code, respectively.
+</p>
+
 <p>
 Finally, the go command now supports packages that import Objective-C
 files (suffixed <code>.m</code>) through cgo.

src/cmd/cgo/doc.go

@@ -63,6 +63,18 @@ compilers may be changed by the CC and CXX environment variables,
 respectively; those environment variables may include command line
 options.
 
+To enable cgo during cross compiling builds, set the CGO_ENABLED
+environment variable to 1 when building the Go tools with make.bash.
+Also, set CC_FOR_TARGET to the C cross compiler for the target.  CC will
+be used for compiling for the host.
+
+After the Go tools are built, when running the go command, CC_FOR_TARGET is
+ignored.  The value of CC_FOR_TARGET when running make.bash is the default
+compiler.  However, you can set the environment variable CC, not CC_FOR_TARGET,
+to control the compiler when running the go tool.
+
+CXX_FOR_TARGET works in a similar way for C++ code.
+
 Go references to C
 
 Within the Go file, C's struct field names that are keywords in Go