fix(chat): elaborate the bidirectionality of the messages array

[ChronosSF]

• Documented messages as a bidirectional collection.
• [Angular] Clarified that Chat mutates the bound collection in place, rather than supporting Angular banana-in-a-box two-way binding syntax.
• Warned against appending the message again in messageCreated.
• Added an array-spread example to preserve the original collection.
• Explained that the shallow copy still shares existing message objects.