Macros-by-example `expr` metavariable substitution behaves as if parenthesized

While reviewing some code, I saw a macro-by-example using a metavariable of type expr in such a way that the code would be wrong if an expression were given to the macro that needed to be parenthesized — so I thought. Testing in the Playground, I found that substituted expressions appeared to be parenthesized automatically. I checked the Reference's section on macros-by-example and saw nothing about this; I looked at the Book's section on macros-by-example and saw nothing about this, but the Book suggested for further reading https://veykril.github.io/tlborm/, which explains that expressions are substituted as AST nodes rather than as tokens and so behave as if parenthesized. I suggest that this be mentioned in the Reference.

Start with the Reference's macros-by-example section and compare its treatment of expr metavariables with the linked Book and TLBORM explanations. Add a note explaining that substituted expressions behave as if parenthesized because they are substituted as AST nodes, and consider the documentation complete when this behavior is described clearly in the Reference.