Use MQTT_PublishToResend() in the basic_tls and mutual_auth demos

[sarenameas]

Description of changes:
For demonstration purposes MQTT_PublishToResend() is now used in the mqtt_basic_tls_demo and the mqtt_mutual_auth_demo. These demos use a Qos > 0.

Extra changes:
Updated the README.md in tools/spell for clarity on how to use the scripts.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #1101 into development will increase coverage by 1.77%.
The diff coverage is 92.42%.

@@               Coverage Diff               @@
##           development    #1101      +/-   ##
===============================================
+ Coverage        96.54%   98.31%   +1.77%     
===============================================
  Files                9        4       -5     
  Lines             5643     1189    -4454     
  Branches           641      369     -272     
===============================================
- Hits              5448     1169    -4279     
+ Misses               9        0       -9     
+ Partials           186       20     -166     

Impacted Files	Coverage Δ
libraries/standard/mqtt/src/mqtt_lightweight.c	96.92% <ø> (+2.40%)	⬆️
libraries/standard/mqtt/src/mqtt_state.c	98.42% <ø> (+2.48%)	⬆️
libraries/standard/http/src/http_client.c	98.53% <80.00%> (+11.05%)	⬆️
libraries/standard/mqtt/src/mqtt.c	99.39% <100.00%> (+4.98%)	⬆️

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[leegeth]

headers

[sarenameas]

Thanks

[aggarw13]

I'm generally concerned that the use of MQTT_PublishToResend seems superfluous when the outgoingPublishPackets array already does the same work of maintaining PUBLISHes for resending?

We probably should show MQTT_PublishToResend in a way that makes its utility more meaningful as the only way of storing PUBLISH packet ID order; otherwise, the use of the utility carries no need for the customer...

[sarenameas]

I'm generally concerned that the use of MQTT_PublishToResend seems superfluous when the outgoingPublishPackets array already does the same work of maintaining PUBLISHes for resending?

We probably should show MQTT_PublishToResend in a way that makes its utility more meaningful as the only way of storing PUBLISH packet ID order; otherwise, the use of the utility carries no need for the customer...

What do you suggest? My understanding is that the application must still own their publish packets and associate them packet IDs.

[aggarw13]

I'm generally concerned that the use of MQTT_PublishToResend seems superfluous when the outgoingPublishPackets array already does the same work of maintaining PUBLISHes for resending?

We probably should show MQTT_PublishToResend in a way that makes its utility more meaningful as the only way of storing PUBLISH packet ID order; otherwise, the use of the utility carries no need for the customer...

What do you suggest? My understanding is that the application must still own their publish packets and associate them packet IDs.

I spoke with @leegeth separately, and after some thinking through, we realized that the outgoingPublishPackets array in the current implementation actually does not preserve the message ordering. Thus, using the MQTT_PublishToResend is a must in these demo applications. Thus, we need to provide the correct documentation to suggest the value using this utility for preserving the message ordering