document joint description of arguments

git-svn-id: https://svn.r-project.org/R/trunk@84739 00db46b3-68df-0310-9c12-caf00c1e9a41

doc/manual/R-exts.texi

@@ -7205,7 +7205,9 @@ Description of the function's arguments, using an entry of the form
 @end example
 
 @noindent for each element of the argument list. (Note that there is
-no whitespace between the three parts of the entry.) There may be
+no whitespace between the three parts of the entry.) Arguments can also
+be described jointly by separating their names with commas (and optional
+whitespace) in the @code{\item} label. There may be
 optional text outside the @code{\item} entries, for example to give
 general information about groups of parameters.