Provide more of an explanation when using function as pattern

[estebank]

(Follow up to #60635 and #60641)

When trying to match on the result of a fn call, we emit:

error[E0164]: expected tuple struct/variant, found method `<A>::new`
  --> src/lib.rs:15:9
   |
15 |         A::new() => (),
   |         ^^^^^^^^ not a tuple variant or struct

We should explain that fn calls are not allowed ever in patterns, only refutable (or even irrefutable, as you can match on a struct, although it is silly) types, possibly linking to (or cribbing text from) the appropriate chapter in The Book.