Or, must it be “Every man is mortal”? How about “Tree is mortal”?
In another sense, “A detailed description of a man”, “A detailed description of man” or “A detailed description of Man”?
Man is a mass noun, therefore it cannot be preceded by an indefinite article. Both man and Man are correct.
Man is mortal.
Beware that if you say:
Every man is mortal.
You are still targeting all humanity, but in that case, the word man has its usual meaning of a person, because you are targeting every man = every person. Therefore, you cannot write this:
Every Man is mortal. (= wrong)
A detailed description of a man.
This sentence is grammatically correct, but it is a bit awkward. The indefinite article says that you are giving a description of a man in general, a person, a human being, but you are also trying to give a detailed description. Therefore, such a sentence would only work in biology, when giving a description about human body, for example.
On the other hand:
A detailed description of the man.
Sounds a lot better and implies that you are going to describe a concrete person in detail.
Last option you presented,
A detailed description of man.
A detailed description of Man.
Both are correct and for the missing article, they imply that you are going to describe whole humanity, which again, in connection with detailed sounds a bit awkward, because it’s very difficult to say what is a detailed description of all humans, but the sentences are grammatically correct.