Explicitly mention bidirectional parameter flow inside variable scoping documentation

[Bertie690]

Currently, the page on scoping within GritQL fails to clearly mention a notable fact about patterns - parameters flow bidirectionally across boundaries.
Any metavariables passed as parameters will be mutated upon returning to the calling scope, a concept that I only discovered after trying random stuff on the playground for 2 hours straight.

As such, I suggest adding an example where a pattern is used to "bind" portions of a larger match to variables at callsites.

Contrived example snippet

// matches an `expect(x).toBe(y)` call and binds the results to `name` and `value`.
pattern match_expect($name, $value) {
    `expect($name).toBe($value)`
}

// n and v are filled in with the values set during match_expect
match_expect(name=$n, value=$v) where {
    $n <: `foo` => `bar`,
    $v <: `baz` => `quux`,
}

converts expect(foo).toBe(baz) ==> expect(bar).toBe(quux)

Note

I put this here mainly for inspiration; you folks can likely come up with much better examples than me.