Clarify hamster cli usage notes

Created by: GeraldJansen

This PR attempts to clarify the usage notes printed by hamster[-cli] -h which are currently somewhat misleading.

For example, the current help implies that one can specify the start and end times, and even though they would be parsed correctly, the program actually only uses the date part and not the HH:MM part.

The message about missing start and end times was also misleading (00:00 to 23:59 instead of using preference for start of hamster day, eg. 05:00).