Add description for faulty formatted timecode test

[KarthikRIyer]

As per discussion here: #753

My rule of thumb is that if a line of code requires a lot of careful explanation when someone new comes across it, it deserves formal documentation at the call site. So @KarthikRIyer I think a couple of sentences that explain to the next person what the test is for are in order.

[meshula]

Thanks!

[ssteinbach]

Great comment, thanks.