docs(realtime): document DataChannels waitForAck param

[harshith-cloudflare]

Summary

• Add waitForAck to OpenAPI DataChannelObject (default: false) and remote request example
• Document gate behavior, default, and that the first subscriber frame is consumed as the ack
• Clarify the channel stays unidirectional; ack is not forwarded
• Add limits note for the 15s acknowledgment timeout and retry guidance

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.

[cloudflare-docs-bot]

Review

❌ Review failed for commit 92415b4. This is usually a transient error — it will retry on the next push.