doc subr rest arg

[Kevin Ryde]

I added a bit to scm_c_define_gsubr about not changing the rest arg.


Existing:
     If RST is non-zero, then any arguments after the first `REQ + OPT'
     are packaged up as a list and passed as FUNCTION's last argument.

Add:

     FUNCTION must not modify that list.  (Because when subr is called
     through `apply' the list comes directly from the `apply' list
     argument.  This list is copied when dispatching to a scheme
     procedure, but not when going to a subr.)