doc: add js/wasm GOOS/GOARCH env pair

Go 1.11 adds an experimental WebAssembly port. The port uses
new GOOS=js GOARCH=wasm values. Some packages, such as syscall/js,
are implemented only for js/wasm environment, so this needs to be
one of the environments to try in order for its documentation to
be available.

Updates golang/gddo#572.

Change-Id: I35b6eae735602e7387beacff07f2b7d6fe323314
Reviewed-on: https://go-review.googlesource.com/131343
Reviewed-by: Tuo Shan <shantuo@google.com>
diff --git a/doc/builder.go b/doc/builder.go
index d0809ca..75ec863 100644
--- a/doc/builder.go
+++ b/doc/builder.go
@@ -465,6 +465,7 @@
 	{"linux", "amd64"},
 	{"darwin", "amd64"},
 	{"windows", "amd64"},
+	{"js", "wasm"},
 	{"linux", "js"},
 }