rustbuild: clarify comment on target array calculation

The comment touched, as originally written, only concerned itself with
the `test` steps. However, since rust-lang#38468 the `arr` variable actually has
gained an indirect relationship with the `dist` steps too. The comment
failed to convey the extra meaning, contributing to the misunderstanding
which eventually lead to rust-lang#38637. Fix that by moving the comment into the
right place near the relevant condition, and properly documenting
`arr`'s purpose.

src/bootstrap/step.rs

@@ -838,9 +838,10 @@ invalid rule dependency graph detected, was a rule added and maybe typo'd?
             } else {
                 &self.build.config.target
             };
-            // If --target was specified but --host wasn't specified, don't run
-            // any host-only tests
+            // Determine the actual targets participating in this rule.
             let arr = if rule.host {
+                // If --target was specified but --host wasn't specified,
+                // don't run any host-only tests
                 if self.build.flags.target.len() > 0 &&
                    self.build.flags.host.len() == 0 {
                     &[]