Clarification on broadcast_time arg for broadcast()

[kevinjwalters]

The broadcast function in adafruit_ble_broadcastnet takes a broadcast_time argument with a default of 0.1 (100ms). The documentation is currently:

Broadcasts the given measurement for the given broadcast time. If extended is False and the
measurement would be too long, it will be split into multiple measurements for transmission.

This doesn't explicitly explain that if the message is split then the function actually takes N * broadcast_time. There's clearly two approaches here, a cap for the overall broadcast time for all "sub" packets or a period per packet to send for. I'm not sure which approach is the best but whatever is intended/implemented should be documented.

This also offers no control over the advertising interval. It's using _ble.start_advertising (note: _). The C code has advertising default interval as (0.1f). If the intent is to send one packet then maybe there's a better way to do that, e.g. 0.020 interval and wait for 0.020 sounds like it achieve the same thing for sending a single packet but that returns at least 80ms earlier to caller offering more opportunity to do useful processing.

[tekktrik]

Not sure if this fixes everything in this issue, but the documentation has been updated to make the timing issue clearer. Let me know if that addresses the scope of this issue!