There is no simple way to set procedure-documentation in guile.
For some years I've done it in the module's scheme description like
(set-procedure-property! proc 'documentation  "help text")

but this is quite hard to maintain, and what seems to be the standard way
(looking at libguile)
is too complicated, so what I did was to redefine the SCM_DEFINE macro
and add a line for setting procedure doc from the included .x file as well.

But, as I can see there is no primitive for setting the procedure doc, only
reading it.
Is it safe to take the  procedure procedure-documentation
and modify the CAR where it expects the doc string to be found?

Of course I can do it by using set-procedure-property from C but I thought
there may be a more direct way.