DOC: ref / val caveat, point at pandas methods
This in part addresses #3340.

I added a few comments in the doc that point users ad using the pandas
at, iat, loc, iloc, etc. methods and included an example similar to
the one exposed in #3340 that addresses some of the reference / value
intricaies encountered with pandas and numpy objects.

To make things clear I selected a rather verbose style.
Let me know if this is over the top ;-)
