cmd/apidiff: add package commentary and move away from os.Args[0]
cmd/apidiff: add package commentary and move away from os.Args[0]

The change regarding os.Args[0] is identicial to the change made in
https://golang.org/cl/145799.

Change-Id: I22e9cef8e5c16504ee71ebe8c9e4183456f0d022
Reviewed-on: https://go-review.googlesource.com/c/152997
Run-TryBot: Paul Jolly <a027184a55211cd23e3f3094f1fdc728df5e0500@myitcv.org.uk>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/exp,golang/exp,golang/exp