Add comment about the manifest's served location

Also for #1278.

src/node/server.ts

@@ -297,6 +297,8 @@ export abstract class Server {
 		switch (base) {
 			case "/":
 				switch (requestPath) {
+					// NOTE: This must be served at the correct location based on the
+					// start_url in the manifest.
 					case "/manifest.json":
 						const response = await this.getResource(this.serverRoot, "media", requestPath);
 						response.cache = true;