FreeRTOS downlink fix

[mcserved]

Summary:

This PR increases the window of the RX retrieval. Additionally it removes the forced ADR sequence and fixes some documentation. Closes #201 #187.

Changes:

• Increase the RX error by 150, allowing the retrieval to start just below 5s.
• Remove the ADR enable sequence at the start, as this was already handled with a variable.
• Improve some documentation.

See the graph here:

basic_lorawan	freertos_lorawan	(new) freertos
5s14ms	5s129ms	4s980ms
218ms	271ms	531ms
781ms	754ms	492ms
218ms	271ms	531ms

Here a picture of the events (compared to the picture in issue #187)

Noticeable from both is that the retrieval times are now within the basic_lorawan application's retrieval window, but also extend far beyond this. So the power consumption is increased by this. I choose 200 specifically as it would be slightly before the 5s mark.

Notes for Reviewers:

Downlinks seem to be better, although not always perfect for me, although this happens with basic_lorawan as well so this can be due to the setup/antenna performance

[elsalahy]

@marnixcro
I think this solves an issue and creates a new one as increasing the Rx window from 200 to 500 is not a welcomed addition from the POV of the LoRaWAN specifications or power consumption.

Please check if we can do anything about this?
Otherwise
Please open a backlog/technical debt issue to reduce the lorawanConfigRX_MAX_TIMING_ERROR and to fix the underlying OS time drift.

[mcserved]

I'll close this PR, and wait for issue #203 to be fixed.