QlikView and Qlik Sense are in many things the very same, for example the script and the behaviour of variables. You are right, the official documenting of features isn't the most strengths of Qlik but the community over compensate many of them with very practically examples.
I think Rob is right you should use the description field of the variable if you want to comment their content.
Using variables in Qlik is in general quite easy but you need to be careful where and how you defined the variables - either script or editor and either with leading equal sign or not respectively the script-versions to them of LET or SET. Further important is which type of content a variable has and where and how they will be called.
This sounds now a bit complex but there are only a few clear rules and the possibility to differ between above mentioned features is a strength and not a bug. By combining several variables within an expression or even using parametrized variables you could easily add a complexity which is hard to understand unless you separate each part of them and looks what really happens in them.
In this sense commentings within variables are just error-prone and you should avoid them.
In your example the important fact is that vTwo starts with an equal-sign and contains therefore the value of 3. The various differences between using the = at the beginning and the use of $-expansion by calling them could you check within some textboxes or maybe a table like: