Add description for USE_TX_BACKPACK user define

[jurgelenas]

See: https://github.com/ExpressLRS/ExpressLRS/blob/master/src/user_defines.txt#L67

[Bryan Mayland]

The USE_ESP8266_BACKPACK should also be removed from user-defines and added to obsolete-defines. Note that the link to obsolete-defines is broken and should link to obsolete-defines but links to Obsolete-user_defines instead so that should be fixed as well.

[Nikhil Parikh]

is this broken on both master & dev or just one?

[Nikhil Parikh]

just dev oops

[Bryan Mayland]

That wasn't for you, I was just making a note for myself when I addressed this issue. Go back to class!

[Nikhil Parikh]

i was doing this real quick haha, i'll stop now

[Bryan Mayland]

Haha you can do it if you want, but it's my define so it's on my to do list to document it. We do have a bunch of doc PRs piled up though kissy face emoji

Edit what! Where did they go? You're the best!

[Bryan Mayland]

Only took me three weeks but I DID IT!