While doing this I found these bugs:

1. A lambda wrapped by cl-function with a :documentation form will
not get a docstring and the :documentation form will be left in the
body of the closure.

2. In code run interactively, a :documentation form in cl-defmacro or
defmacro will work correctly. However if you byte-compile that code,
the byte-compiler stops with the following error message:
    Symbol’s function definition is void: internal-make-closure

3. A :documentation form in cl-defgeneric only works if its argument
is a string literal. In cl-defmethod, it works whether the argument is
a string literal or an expression.

4. A :documentation form works in cl-iter-defun and iter-defun, but
not in iter-lambda.

Stefan, here's a list of macros in which it appears to me that
(:documentation FORM) is supposed to work. Is this correct? Am I
missing any?

lambda
pcase-lambda
defun
defmacro
cl-defun
cl-defmacro
iter-defun
cl-iter-defun
iter-lambda
function
cl-function
cl-defgeneric
cl-defmethod
defsubst
cl-defsubst