Clarify comment on update_existing_submodules

This felt like an important point to me.

Author: ehuss

src/bootstrap/src/lib.rs

@@ -583,7 +583,8 @@ impl Build {
     /// If any submodule has been initialized already, sync it unconditionally.
     /// This avoids contributors checking in a submodule change by accident.
     fn update_existing_submodules(&self) {
-        // Avoid running git when there isn't a git checkout.
+        // Avoid running git when there isn't a git checkout, or the user has
+        // explicitly disabled submodules in `config.toml`.
         if !self.config.submodules(self.rust_info()) {
             return;
         }