Document start & stop commands

After installing timewarrior, I wanted to start timetracking with the `start` command, so I simply tried to execute `task 1 start 9:00`, assuming the task would thus be marked as started from 9 in the morning. But after some experimentation I realized that this merely created an annotation and started the task from now. After further experiments,it seems to be that
`task 1 start ` behaves exactly the same as `task 1 annotate start:now`
That also means that `task 1 start:2020-05-06T09:00` and `task 1 start start:2020-05-06T09:00` both don't work, which means the only way to start a task in the past is `task 1 mod start:2020-05-06T09:00`.

This should definitely be documented, and if possible adjustments made so a task start time can be set easier within the command.