init kwarg on sum!, any!, prod!, etc not documented

[oxinabox]

A bunch of our special reduction functions with mutating forms, like sum!, any! and prod! have an init keyword argument.
I saw i used in the wild today and i had no idea what it did (well i could guess, but I wanted to check the docstring).

julia> sum!([10, 20], [1 2; 3 4])
2-element Vector{Int64}:
 3
 7

julia> sum!([10, 20], [1 2; 3 4], init=false)
2-element Vector{Int64}:
 13
 27

[mcabbott]

Guilty as charged. And xref #36266 for this.

[simeonschaub]

This may also be on purpose, see #36305