DOC: Add section on using in operator with Series, DataFrame
closes #5571

Revised. Thanks, @jorisvandenbossche .
 can u add a reference to this section
in basics.rst where series is talked about as dict-like 

thanks
 @jorisvandenbossche looks fine to me
 I haven't got sphinx working on pandas locally, so I haven't verified this. But the issues discussed above have been fixed.
 Looking good (and you can always see the result after a while on http://pandas-docs.github.io/pandas-docs-travis/ to check if everything is OK). Thanks!
