flag: Document that FlagSet.Arg(int) and Arg(int) returns empty string if arg doesn't exist

[srinathh]

I thought it wasn't obvious these functions should return empty string rather than have some other behavior like panic (as would happen when trying to access a non-existent member of os.Args[])

[gopherbot]

CL https://golang.org/cl/12061 mentions this issue.

[srinathh]

I sent a tiny documentation change for this in https://go-review.googlesource.com/12061 - thought i'd take a shot at it to get a hang of how the contribution process works :-)