Better Documentation for hs.fs.dir()

[latenitefilms]

Based on the API documentation it's not clear that hs.fs.dir() will return a nil value for the iterator function if something doesn't work. For example:

> iterFn, dirObj = hs.fs.dir("/Users/Guest/Documents")
print(string.format("iterFn: %s", iterFn))
print(string.format("dirObj: %s", dirObj))
2018-08-28 12:11:11: iterFn: nil
2018-08-28 12:11:11: dirObj: cannot open /Users/Guest/Documents: Permission denied

This becomes problematic when you're using a for loop, for example:

for file in hs.fs.dir("/Users/Guest/Documents") do 
	print(file)
end

Maybe either the documentation or the function itself needs to be re-thought @cmsj or @asmagill ?

[cmsj]

I suspect probably the docs should include an example

[latenitefilms]

Fixed in #1928