enum.c patch: fixes Enumerable.cycle and rdoc bugs

The attached patch fixes:

1) The rdoc for find_index described a method argument that does not 
exist, and also showed the wrong return value in one of the examples

2) The rdoc for take and drop were unclear: they made them sound kind of 
like synonyms for select and reject, and the examples did not illustrate 
the difference

3) The implementation of Enumerable.cycle was subject to the same bug 
that I previous reported for Array.cycle: when called on an empty 
collection, it would spin forever within tight infinite loop, never 
invoking the associated block, and requiring a kill -9 as the only way 
to terminate the process.  The patch makes cycle return nil if the 
Enumerable has no elements, and updates the documentation to reflect that.

	David