some (minor) Rd issue: line breaking in \S4method within \usage{}

Hi,

this is perhaps not so much an issue, but as you are currently
working on the Rd parser anyway...

When you want to document specific S4 methods for an S4 generic
by \S4method{} within a \usage{} environment, and the second
(signature_list) argument is "long", be it because you use multiple
dispatch, be it because you use long class-names, (or both), one
would probably like to be able to insert line breaks within the
signature list [or at least insert line breaks at suitable places
in the corresponding generated pdf file].

AFAICS this is not possible so far, at least I get errors in
R CMD check, whenever I try something like

\S4method{<myMethod>}{<ClassOf1stArgToDispatch>,
<ClassOf2ndArgToDispatch>}(<Name1stArgToDispatch>,<Name2ndArgToDispatch>)

i.e. if I insert a line break in the Rd file after any item of the
signature_list argument to \S4method.

Using line breaks in the the third (argument_list) argument is not a
problem, though.

So my little RFE would be to ask you to allow some optional line
breaking mechanism for the second argument of \S4method, too,
if this is not too hard to achieve.