[Rd] Documentation of ``Extract'' re. drop argument (PR#4297)

[Peter Dalgaard BSA]

david.meyer at ci.tuwien.ac.at writes:

> Adding a corresponding note in the doc. of the `drop' argument, or 
> after the
> 
>   If one of these expressions appears on the left side of an
>   assignment then that part of 'x' is set to the value of the right
>   hand side of the assignment.
> 
> paragraph in the `details' section of the `Extract' help page.
> 

I'm fixing this and documenting logical and matrix indexing while I'm
there. 

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
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