Clarify MQTT tenant-id usage for TTN V3

[mtxgithub]

Summary

The Documentation for MQTT can be misleading about required tenant-id usage for TTN V3.
Adding some info can clarify it.

Why do we need this ?

Easier to understand on first usage; background in the Forum:
https://www.thethingsnetwork.org/forum/t/downlink-formatter/45359/8

What is already there? What do you see now?

https://www.thethingsindustries.com/docs/integrations/mqtt/

What is missing? What do you want to see?

MQTT Server -> "Multi-Tenancy"

"Note that on multi-tenant environments such as Cloud, application IDs and other endpoints include the tenant ID, e.g app1@tenant1. On single tenant environments such as Open Source, the tenant ID can be removed, i.e app1."

"tenant ID can be removed" is misleading if you read this for the first time, better add something like:
"The Things Network V3" requires including the tenant ID "@ttn"

"Integration with MQTT Clients":

"Eclipse Mosquitto":

(typo: replace /up by /push): in the example
mosquitto_pub -h "thethings. ... -t "v3/app1/devices/dev1/up" ....
(better mention the requirement of the tenant ID as well)

"Eclipse Paho":

Adopted the example / working would be this one (...adding the tenant-id again):

publish.single("v3/{application-id}@{tenant-id}/devices/{device-id}/down/push", '{"downlinks":[{"f_port": 15,"frm_payload":"vu8=","priority": "NORMAL"}]}', hostname="thethings.example.com", port=1883, auth={'username':"{application-id}@{tenant-id}",'password':"NNSXS.VEEBURF3KR77ZR.."})

How do you propose to document this?

mentioned above.

Can you do this yourself and submit a Pull Request?

no

[nejraselimovic]

Thank you for your contribution @mtxgithub!