`@distributed` fails on loops over iterators

Distributed for loops over iterators fail, seemingly because @distributed expects the iterator to implement getindex. It makes sense that @distribute needs getindex, but I don't see that in the docs. The possible solutions seem to be:

1. Update the docs for @distributed to mention this requirement.
2. Update the implementation of @distributed to work for iterators.

We could do JuliaLang/julia#1 until JuliaLang/julia#2 is finished.

Here's an example:

@distributed (*) for (i, j) in Base.Iterators.product([1, 2], [3, 4])
    i + j
end

ERROR: LoadError: MethodError: no method matching getindex(::Base.Iterators.ProductIterator{Tuple{Array{Int64,1},Array{Int64,1}}}, ::Int64)
(::getfield(Main, Symbol("##7#8")))(::typeof(+), ::Base.Iterators.ProductIterator{Tuple{Array{Int64,1},Array{Int64,1}}}, ::Int64, ::Int64) at /home/dan/julia/usr/share/julia/stdlib/v1.0/Distributed/src/macros.jl:275
(::getfield(Distributed, Symbol("##143#144")){getfield(Main, Symbol("##7#8")),Tuple{typeof(+),Base.Iterators.ProductIterator{Tuple{Array{Int64,1},Array{Int64,1}}},Int64,Int64},Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}})() at /home/dan/julia/usr/share/julia/stdlib/v1.0/Distributed/src/remotecall.jl:339
run_work_thunk(::getfield(Distributed, Symbol("##143#144")){getfield(Main, Symbol("##7#8")),Tuple{typeof(+),Base.Iterators.ProductIterator{Tuple{Array{Int64,1},Array{Int64,1}}},Int64,Int64},Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}}, ::Bool) at /home/dan/julia/usr/share/julia/stdlib/v1.0/Distributed/src/process_messages.jl:56
#remotecall_fetch#148(::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}, ::Function, ::Function, ::Distributed.LocalProcess, ::Function, ::Vararg{Any,N} where N) at /home/dan/julia/usr/share/julia/stdlib/v1.0/Distributed/src/remotecall.jl:364
remotecall_fetch(::Function, ::Distributed.LocalProcess, ::Function, ::Vararg{Any,N} where N) at /home/dan/julia/usr/share/julia/stdlib/v1.0/Distributed/src/remotecall.jl:364
#remotecall_fetch#152(::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}, ::Function, ::Function, ::Int64, ::Function, ::Vararg{Any,N} where N) at /home/dan/julia/usr/share/julia/stdlib/v1.0/Distributed/src/remotecall.jl:406
remotecall_fetch at /home/dan/julia/usr/share/julia/stdlib/v1.0/Distributed/src/remotecall.jl:406 [inlined]
(::getfield(Distributed, Symbol("##167#168")){typeof(+),getfield(Main, Symbol("##7#8")),Base.Iterators.ProductIterator{Tuple{Array{Int64,1},Array{Int64,1}}},Array{UnitRange{Int64},1},Int64,Int64})() at /home/dan/julia/usr/share/julia/stdlib/v1.0/Distributed/src/macros.jl:259
Stacktrace:
 [1] try_yieldto(::typeof(Base.ensure_rescheduled), ::Base.RefValue{Task}) at ./event.jl:196
 [2] wait() at ./event.jl:255
 [3] wait(::Condition) at ./event.jl:46
 [4] wait(::Task) at ./task.jl:188
 [5] fetch at ./task.jl:202 [inlined]
 [6] iterate at ./generator.jl:47 [inlined]
 [7] collect(::Base.Generator{Array{Task,1},typeof(fetch)}) at ./array.jl:619
 [8] preduce(::Function, ::Function, ::Base.Iterators.ProductIterator{Tuple{Array{Int64,1},Array{Int64,1}}}) at /home/dan/julia/usr/share/julia/stdlib/v1.0/Distributed/src/macros.jl:263
 [9] top-level scope at /home/dan/julia/usr/share/julia/stdlib/v1.0/Distributed/src/macros.jl:274
 [10] include at ./boot.jl:317 [inlined]
 [11] include_relative(::Module, ::String) at ./loading.jl:1044
 [12] include(::Module, ::String) at ./sysimg.jl:29
 [13] include(::String) at ./client.jl:392
 [14] top-level scope at none:0
in expression starting at /home/dan/julia/test.jl:3

This is surprising, since the following non-parallel reduction works:

julia> reduce((*), [i + j for (i, j) = Base.Iterators.product([1, 2], [3, 4])])  
600

Adding a collect to the distributed version fixes it:

@distributed (*) for (i, j) in collect(Base.Iterators.product([1, 2], [3, 4]))
    i + j
end

Output: 600

Julia Version 1.0.2
Commit d789231e99* (2018-11-08 20:11 UTC)
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-6.0.0 (ORCJIT, skylake)
Environment:
  JULIA_REVISE_INCLUDE = 1
  JULIA_DEBUG = all

Start by reading Distributed/src/macros.jl around lines 259-275 and reproduce the failure with Base.Iterators.product from the issue. Compare the failing iterator case with the working collect-based example, then determine whether the intended result is implementation support or documentation of the getindex requirement. Done means the chosen behavior is explicit and the reported example is handled or clearly documented.