Common Lisp: Getting the documentation string of a macro

In SBCL, I can get the documentation string for a function with something like this:

(documentation #'mapcar t)

However, I don't understand how to get the documentation string for a macro. For example, given the macro:

(defmacro with-lines-in-file ((line filename) &body body)
  "Runs body for each line in the file specified by filename."
  (let ((file (gensym)))
    `(with-open-file (,file ,filename)
      (do ((,line (read-line ,file nil) (read-line ,file nil)))
          ((null ,line) nil)
        ,@body))))

I'm not able to retrieve the documentation string. I don't understand the CLHS. As you can see below, the CLHS worked great for obtaining the documentation string for a function.

documentation (x function) (doc-type (eql 't))

However, the bit for obtaining the documentation string from a macro doesn't seem to work for me:

documentation (x symbol) (doc-type (eql 'compiler-macro))

In the context of my macro, I'm interpreting the above CLHS bit to mean this:

(documentation 'with-lines-in-file 'compiler-macro)

But that call returns NIL.

I'm trying to build a function that creates the README.md file for the Common Lisp packages that I plan to share on GitHub. Here's an example: https://github.com/macnod/dc-utilities. I've written a post about this here: https://donnieknows.com/documenting-common-lisp-for-github/.