Improve clock description

[behrtam]

What is a valid input? 20 s? 3:20 pm? half past noon? Once we have the input, we need to be able to "add and subtract minutes to it". What are we supposed to return? 1 m 20 s? 3:21 pm? half past noon and one minute?

Maybe this (https://github.com/exercism/xpython/issues/401) is better handled here.

[ErikSchierboom]

Admittedly, the current description is fairly succinct.

# Description

Implement a clock that handles times without dates.

You should be able to add and subtract minutes to it.

Two clocks that represent the same time should be equal to each other.

We should probably explicitly mention that a clock has an hour and minute component. The instructions should also mention what happens with "overflows".