Forget "markup languages" like markdown. Forget "auto-doc" languages like Doxygen. Forget MSWord.

You're not writing "documentation", you're communicating to another person or, more likely, your future self.

In fact, what you WANT to write is a Literate Program. http://www.literateprogramming.com/

Literate programs have the actual source code embedded in the actual document. You extract the source code from the document and run it. It is always up to date.

Think about a physics textbook. All of the equations are "source code", all of the paragraphs are explanation. Could you learn physics if you just had the equations without the words? Then why would you expect anyone to maintain and modify your program with "only the equations", and no explanations (note: NOT documentation).

Some excellent literate programs are:

Matt Pharr and Greg Humphries. "Physically Based Rendering: From Theory to Implementation", Morgan Kaufmann, 2004

Christian Queinnec "Lisp in Small Pieces" Cambridge University Press (1996) 978-0521545662

Latex is the language of science communication. Learn it. Live it.

Latex is a "must know" language.