docs(aio): Clearing array with [] (#20369) (#20395)

Clearing array with setting length to 0 replaced with [] for being short
and marginally efficient. For reference: [] is turned into a sequence of
around 15 machine instructions on x64 (if bump pointer allocation succeeds),
whereas a.length=0 is a C++ runtime function call, which requires 10-100x as
many instructions.
Benedikt Meurer

PR Close #20395

aio/content/examples/toh-pt4/src/app/message.service.ts

@@ -9,6 +9,6 @@ export class MessageService {
   }
 
   clear() {
-    this.messages.length = 0;
+    this.messages = [];
   }
 }