See, you already have it,
Self documenting code uses well chosen variable names (and function names) to make the code read as close to English as possible. This should be your goal.
For example, naming a variable g has little meaning, but naming a variable gravity gives a much better description of what the variable should contain.
By using proper variable and function names, you should minimize the amount of "external" documentation that is necessary. For example, compare the following two pieces of code?
