Document `op` argument order in `foldl`

[jariji]

foldl(op, itr; [init])

Like reduce, but with guaranteed left associativity. If provided, the keyword argument init will be used exactly
once. In general, it will be necessary to provide init to work with empty collections.

See also mapfoldl, foldr, accumulate.

Examples
≡≡≡≡≡≡≡≡

julia> foldl(=>, 1:4)
((1 => 2) => 3) => 4

julia> foldl(=>, 1:4; init=0)
(((0 => 1) => 2) => 3) => 4

julia> accumulate(=>, (1,2,3,4))
(1, 1 => 2, (1 => 2) => 3, ((1 => 2) => 3) => 4)

I don't think it's clear from this whether op will always receive (accumulator, x) or (x, accumulator). The answer is (accumulator, x) which is necessary in order to support asymmetric ops like push!:

julia> foldl(push!, 1:2; init=Int[])
2-element Vector{Int64}:
 1
 2

We might guess that's what it means from the examples, but you can't really infer it's a guarantee rather than a current implementation detail.