doco append

[Kevin Ryde]

A bit of a revision for append and append!,

        * scheme-compound.texi (Append/Reverse): Merge append and append!,
        show parameters as lst1 ... lstN, describe list argument for
        scm_append and scm_append_x and note that it's unmodified.


 - Scheme Procedure: append lst1 ... lstN
 - Scheme Procedure: append! lst1 ... lstN
 - C Function: scm_append (lstlst)
 - C Function: scm_append_x (lstlst)
     Return a list comprising all the elements lists LST1 to LSTN.

          (append '(x) '(y))          =>  (x y)
          (append '(a) '(b c d))      =>  (a b c d)
          (append '(a (b)) '((c)))    =>  (a (b) (c))

     The last argument LSTN may actually be any object; an improper
     list results if the last argument is not a proper list.

          (append '(a b) '(c . d))    =>  (a b c . d)
          (append '() 'a)             =>  a

     `append' doesn't modify the given lists, but the return may share
     structure with the final LSTN.  `append!' modifies the lists to
     form its return.

     For `scm_append' and `scm_append_x', LSTLST is a list of the lists
     LST1 ... LSTN.  That LSTLST itself is not modified or used in the
     return.