doc reverse! newtail

[Kevin Ryde]

        * scheme-compound.texi (Append/Reverse): Merge reverse and reverse!,
        describe newtail parameter for reverse!, remove confusing caveat about
        head becoming tail for reverse!.

(And I think the docstring for reverse! can be modified similarly.)


 - Scheme Procedure: reverse lst
 - Scheme Procedure: reverse! lst [newtail]
 - C Function: scm_reverse (lst)
 - C Function: scm_reverse_x (lst, newtail)
     Return a list comprising the elements of LST, in reverse order.

     `reverse' constructs a new list, `reverse!' modifies LST to
     construct its return.

     For `reverse!', the optional NEWTAIL is appended to to the result.
     NEWTAIL isn't reversed, it simply becomes the list tail.  For
     `scm_reverse_x', NEWTAIL is mandatory, but can be `SCM_EOL' if no
     further tail is required.