Docs feedback: let! are not referenced

Let variables are the main subject in this paragraph of testing best practices docs. Unfortunately, this document references only lazy version of let, leaving strict version let! behind. To fully cover the subject it is better to supply some clarifications on how to use the strict version, if in should be avoided, used only in special contexts or else.

I'd suggest such refinements:

let variables are preferable to instance variables. Local variables are preferable to let variables.

This line may be changed as follows: let variables are preferable to let! variables, let! variables are preferable to instance variables. Local variables are preferable to let variables.

Also, such line may be added: let! variables should be used only in case if strict evaluation with defined order is required, otherwise let will suffice. Remember that let is lazy and won't be evaluated until it is directly referenced.