If you write Emacs functions—even for yourself—you are, of course, including a docstring. Functions have a way of escaping from your init.el and finding their way to someone else’s config. Even if that doesn’t happen you may find yourself happy to have a reminder of the function’s details. At least if you’re like me.
It turns out that docstrings have their own mini-markup language. Xah Lee has a nice tutorial on how to use that markup language. I find Lee’s post clearer and easier to read than the Documentation Tips page of the Elisp Manual.
The Emacs built-in documentation is nicely formatted and, when properly done, very useful. Most times, it’s all you need to use a command, function, or variable successfully. By learning and using the markup language that docstrings provide, you can document your code as nicely as the built-in functions.