doc: document GODEBUG=netdns=X and netcgo in go1.5.txt

Change-Id: I720aeb1511e407750617e23c4cba1edcddf745bb
Reviewed-on: https://go-review.googlesource.com/12326Reviewed-by: Rob Pike <r@golang.org>

doc/go1.5.html

@@ -711,6 +711,11 @@ system resources.
 The decision of how to run the resolver applies at run time, not build time.
 The <code>netgo</code> build tag that has been used to enforce the use
 of the Go resolver is no longer necessary, although it still works.
+A new <code>netcgo</code> build tag forces the use of the <code>cgo</code> resolver at
+build time.
+To force <code>cgo</code> resolution at run time set
+<code>GODEBUG=netdns=cgo</code> in the environment.
+More debug options are documented <a href="https://golang.org/cl/11584">here</a>.
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