docs: add some clarifications to methods.md

[vtjnash]

Closes #40650
Rebase of #40667 with edits, since upstream deleted their repo

[mpeters2]

Instead of using single-line functions for your examples, can you use multiline function definitions when possible? For those of us who do not live in the REPL, those types of examples can be confusing, as they do not follow our usual coding style.

For example, instead of this (which is a function that doesn't do anything, or at least it is non-obvious):

julia> bar(a, b, x...) = (a, b, x)
bar (generic function with 1 method)

this:

function bar(a, b, x...)
    println("a = ", a)
    println("b = ", b)
    println("x... = ", x)
end

julia> bar(1, 2, 3, 4)
a = 1
b = 2
x... = (3, 4)