cmd/go: rename doc.go to alldocs.go in preparation for "go doc"

Also rename and update mkdoc.sh to mkalldocs.sh

Change-Id: Ief3673c22d45624e173fc65ee279cea324da03b5
Reviewed-on: https://go-review.googlesource.com/9226Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/mkdoc.sh → src/cmd/go/mkalldocs.sh

@@ -6,7 +6,7 @@
 set -e
 
 go build -o go.latest
-./go.latest help documentation | sed 's; \*/; * /;' >doc.go
-gofmt -w doc.go
+./go.latest help documentation | sed 's; \*/; * /;' >alldocs.go
+gofmt -w alldocs.go
 rm go.latest