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.

CLN: cleanup + edits - addresses recent discussion

CLN: cleanup II

CLN: wrap at 80 chars.

took care of both documents.
Merge pull request #3399 from dengemann/ref_value_data_access_doc

DOC: ref / val caveat, point at pandas methods - thanks!