Clarify list pattern behavior in documentation

[panpanpro888]

I was reading the Gleam language overview on list patterns and it's not clear that you can store the matched values in variables. While this is seen 3 sections below on the list recursion section, its main point is not to show how list patterns function, so I think it would be nice to have it in the documentation. Something like:

You can also store these values in variables. [first, second, ..rest] will match the first two values in first and second respectively and the rest in rest

And in the examples:

import gleam/string
import gleam/int
import gleam/list

[3] -> "List of just 3"
[3, ..rest] -> "List starting with 3 and the rest being: " <> rest |> list.map(fn(n) { int.to_string(n) }) |> string.join(", ")