Clarify/Fix capturing/reading of non-modulated IR signals

Clarify what happens when IrWidget sees a non-modulated signal. What exactly does f=20000 mean? Suggest a way for capuring/reading of such signals.

Start by reviewing how Infrared4Arduino captures and reads non-modulated IR signals, then determine what the f=20000 value represents. Define the expected behavior for such signals and document or propose a capture approach; the issue does not name specific files or tests, so repository exploration is required.

Written by the indexing model from the issue text.