Feature request: Update code samples in broadcast.md to use the 'backoff' sending method

[bwcDvorak]

Is your feature request related to a problem?

Document text has been updated to describe the preferred method of bulk sending, which prioritizes reliability over raw speed and will work better for the intended audience. The corresponding sample code, however, has not been update.

Describe the solution to the problem

Update the following samples:

• Javascript
• React Native

Describe the uses cases for the feature

Developers will more easily and more effectively be able to make use of bulk sending

Additional details

No response