Nothing changes when the value is invalid. A knob's id is its dotted path into the module's
configuration, so the change also shows in getConfig and fires a config-change event. To
set several knobs in one change, use setKnobs.
Use it for a value held as data, from a control, storage or a language model, since it checks the
value and keeps the settings beside it. For typed settings your code writes, including the ones no
knob covers, use the module's applyConfig or updateConfig, which check nothing at run time.
Sets one knob by its catalogue id, after checking the value with validateKnobValue, and keeps every other setting.