Further review feedback re: overrides

ensure.go

@@ -65,7 +65,9 @@ dep ensure github.com/pkg/foo:git.internal.com/alt/foo
 dep ensure -override github.com/pkg/foo@^1.0.1
 
     Forcefully and transitively override any constraint for this dependency.
-    This can inadvertantly make your dependency graph unsolvable; use sparingly.
+    Overrides are powerful, but they are harmful over the long term. They should
+    be used as a last resort, especially if your project will be imported by
+    others.
 `
 
 func (cmd *ensureCommand) Name() string      { return "ensure" }