isnull() and notnull() - documentation
Hi,

It will be nice if someone can add that
`isnull()` function have a contrary which is called `notnull()`
and
`notnull()` function have a contrary which is called `isnull()`
into documentation

http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.notnull.html
http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.isnull.html

That's probably stupid for you but I was searching a long time to find that
(I was looking for `isnotnull()`)

Maybe this documentation could also tell that

`df[df['col'].notnull()]` is same as ``df[~df['col'].isnull()]`

Kind regards
