add missing let blocks and array comprehensions

[gdkrmr]

I think these were missing from the documentations. I am not sure about the proper indentation.

[iamed2]

Is there a reason let would need to be used rather than begin? If so, could you include that reasoning?

[tknopp]

begin is not valid in 0.7/1.0

[gdkrmr]

I personally have no idea why it has to be a let block now. I tried to update some code to 0.7 and failed because there was no reference in the manual, so I decided to add it.

[iamed2]

begin is not valid in 0.7/1.0

This works:

julia> [ begin
           x = 2
           i + x
         end for i=1:10 ]
10-element Array{Int64,1}:
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12

[gdkrmr]

you are right, let and begin work now without a warning in both

Julia Version 0.7.1-pre.0
Commit 36cddc1 (2018-08-09 00:19 UTC)

and

Julia Version 1.0.0
Commit 5d4eaca (2018-08-08 20:58 UTC)

[JeffBezanson]

Yeah, I don't really love how we no longer allow begin in indexing expressions. Every expression should be allowed everywhere if at all possible. There just hasn't been a good alternate proposal for how to reference first and last indices.