cmake: Clarify documention regarding libqrencode optional dependency

[hebasto]

This PR addresses this feedback:

It seems that the bitcoin-qt qrencode dependency default value deviates from the one in autotools. I don't have the qrencode library available in my system and I'm able to compile the gui out of the box through autotools but not through cmake due to the missing dependency. It would be nice to use the same default value for now. And discuss any possible default value change (if needed) in a follow-up.

[hebasto]

cc @furszy @theStack

[furszy]

all good for me.

[m3dwards]

LGTM!