DOC: Distinguish between different types of boolean indexing
It's not clear from the docs that indexing with a boolean `ndarray` isn't the same as indexing with a boolean `Series`. It took me a while to realise that:

``` python
import pandas as pd
df = pd.DataFrame([[1, 2], [3, 4], [5, 6]], list('abc'), ['one', 'two'])
sr = (df['one'] > 2)
df.loc[sr, 'two']      # This is ok
df.iloc[sr, 1]         # But this is not
df.iloc[sr.values, 1]  # The right way to use iloc
```

I've since read through #3631 and it makes sense to me. However, even though the docs emphasise that `.iloc` is **purely integer-based**, it didn't click at the time that indexability of `Series` was the problem I was facing.
