Issue 4121/2: Store markup command argument list names in docstring

Issue 4121/2: Store markup command argument list names in docstring

For one thing, the doc string does not make a lot of sense without the
argument list and it cannot reliably be deduced from the doc string
itself.

For another, GUILEv2 does not do us the favor of storing the original
argument list anywhere where it could be retrieved with a useful amount
of labor.

While it would be nice to put the types of the arguments into the doc
string as well, the signature's actual predicates are only evaluated
after macro expansion and that is too late for generating the doc
string.

So we just stick the argument list (after uncurrying) as a line at the
top.