add docstrings to cwstring and transcode

[simonbyrne]

No description provided.

[Keno]

Should this mention that this is supposed to be used via Cwstring?

[simonbyrne]

Is it? From what I can tell, the following are equivalent:

ccall(....,Cwstring, str)

or

cwstr = cwstring(str)
ccall(....,Ptr{Cwchar_t}, cwstr)

the advantage of the 2nd one is that if you have to call the function multiple times, you only have to do the conversion once.

[Keno]

I thought Cwstring was exported, but this was not.

[ararslan]

Looks like @Keno is right: Cwstring appears in base/exports.jl but not cwstring.

[simonbyrne]

It isn't exported, but is that any reason not to document it?

[simonbyrne]

I realise these functions aren't exported, but probably good to have documented (we may want to export transcode at some point).

The only thing that isn't documented is the recommended way to get a String back from a ccall that returns a UTF-16 string. The examples in Base all use String(Base.transcode(UInt8, x)), where x is a Vector{UInt16}.

[tkelman]

Can anyone explain the justification for only defining cwstring on Windows?

[StefanKarpinski]

Can anyone explain the justification for only defining cwstring on Windows?

The definition of cwstring doesn't work on systems where Cwchar_t != UInt16 since we don't currently define transcode methods for UTF-32. I agree that this should be simplified, but that doesn't block this PR.