Extend comment on progress bar example

Adds a note to the progress bar example on how HourGlass may be used in the case that there's no easy way to calculate the progress, or when showing a progress bar is not worth it.

This way, it encourages developers to not make their programs unresponsive just because showing a progress bar is not feasible. The HourGlass doesn't speed down the calculation much, if called with the proper frequency (about twice per second, and with my Eigenmath port I noticed that even calling it ten times per second while doing keyboard reading doesn't have much impact).