pmap vs map for DArray?

Searching through the source for `DistributedArray`, it seems I'm supposed to use `map` instead of `pmap` on a `DArray`. Indeed, when I try to use `pmap`, strange things happen:

```
julia> testa = drand(36)
36-element DistributedArrays.DArray{Float64,1,Array{Float64,1}}:
0.636737
0.275769
0.961624
0.427848
0.668537
0.0215699
0.292591
0.487622
0.54222
0.579438
0.299413
0.0699156
0.985861
0.642223
0.0108336
0.466572
0.134984
0.0718047
0.600704
0.367337
0.722101
0.96763
0.427482
0.963513
0.467348
0.987774
0.773584
0.531576
0.0155698
0.383172
0.0347603
0.299581
0.0226568
0.687901
0.22271
0.238291

julia> testb = drand(36)
36-element DistributedArrays.DArray{Float64,1,Array{Float64,1}}:
0.933919
0.445692
0.0028197
0.722083
0.088373
0.820338
0.71782
0.972424
0.623399
0.157076
0.657007
0.0753378
0.712683
0.303925
0.591726
0.320129
0.5457
0.00830437
0.0753483
0.973917
0.171903
0.291315
0.875653
0.0619788
0.53868
0.069952
0.534305
0.798335
0.923633
0.239445
0.748613
0.00554521
0.650063
0.770877
0.237519
0.414616

julia> pmap( (x) -> (x*x), testa)
175-element Array{Any,1}:
0.405433
0.405433
0.405433
0.405433
0.405433
0.405433
0.0760486
0.0760486
0.0760486
0.0760486
0.0760486
0.0760486
0.92472
0.92472
0.92472
0.183054
0.183054
0.183054
0.92472
0.183054
0.183054
0.183054
0.446942
0.446942
0.446942
0.446942
0.000465259
0.446942
⋮
0.00120828
0.00120828
0.00120828
0.00120828
0.00120828
0.00120828
0.0897485
0.0897485
0.0897485
0.0897485
0.0897485
0.000513332
0.000513332
0.473207
0.000513332
0.000513332
0.473207
0.473207
0.473207
0.0495998
0.473207
0.0495998
0.0495998
0.0495998
0.0567826
0.0567826
0.0567826
0.0567826

julia> map( (x) -> (x*x), testb)
36-element DistributedArrays.DArray{Float64,1,Array{Float64,1}}:
0.872205
0.198641
7.95072e-6
0.521403
0.00780978
0.672954
0.515266
0.945609
0.388626
0.0246728
0.431658
0.00567578
0.507917
0.0923703
0.35014
0.102483
0.297789
6.89625e-5
0.00567737
0.948514
0.0295506
0.0848642
0.766769
0.00384137
0.290177
0.00489328
0.285482
0.637339
0.853098
0.0573337
0.560421
3.07494e-5
0.422581
0.594251
0.0564152
0.171907
```

(Sorry for how long that was!)

Yet intuitively, I would have expected `pmap` to work like `map` does on a distributed array. I'm fine with my intuition being wrong here, but should there be a note in the documentation to use `map`, and not `pmap`? What is the expected behaviour of `pmap` on a `DArray`?

Start by reproducing the issue's `drand`, `map`, and `pmap` examples, then search the repository for `DArray`, `map`, and `pmap` documentation or entry points. Determine the expected behavior of `pmap` on a `DArray` and document the distinction from `map`, including a concise example or limitation if supported by the existing behavior.

Written by the indexing model from the issue text.