If such a macro is used in more than two places, it's generally a relief that the author did that instead of doing copy and paste by hand.
The author had some coding idea, and formalized it into a little robot that writes that idea, which has a name.
Even if that thing isn't accompanied by documentation, it can serve as a kind of documentation to what it's doing.
Code is going to be full of homemade functions that are not documented; whether they are functions that write code at compile time, or whether they are run-time functions, is kind of a minor concern.
Most macros are just simple syntactic abstractions that expands simple code to more lines of relatively simple code to avoid boilerplate. Very rarely do people write complex beasts without documentation.