cmd/goimports: update docs to not imply goimports is superset of gofmt
cmd/goimports: update docs to not imply goimports is superset of gofmt

Change-Id: I098fa50c274d8206b6e2534d647cfae18d6b2703
Reviewed-on: https://go-review.googlesource.com/45390
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

syreclabs/go-tools,golang/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,golang/tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools