Brevity is the soul of wit
This phrase means that humour should be kept short and to the point otherwise it might lose its effect. However, it can also refer to a piece of writing, in the same way it does with humour in that the writing should be concise in order to best display its meaning.
You could use this phrase when a person with whom you are talking goes above and beyond the point that they are trying to make. If that person is attempting to over-explain, you might use this term.