Document parameters

[zsup]

I went to run spark cloud flash blinky.ino and it told me that I had to pick a source file. Actually what had happened was I hadn't added a core ID, but it thought that blinky.ino was the binary.

In addition to the error message, it should return full documentation for the function, so that I could catch this error by seeing something like:

spark cloud flash <<core-id>> <<source-file>>