DOC: Improve DataFrame.dropna subset example
In the examples for [DataFrame.dropna](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.dropna.html), which begin like so:

```pycon
>>> df = pd.DataFrame({"name": ['Alfred', 'Batman', 'Catwoman'],
...                    "toy": [np.nan, 'Batmobile', 'Bullwhip'],
...                    "born": [pd.NaT, pd.Timestamp("1940-04-25"),
...                             pd.NaT]})
>>> df
       name        toy       born
0    Alfred        NaN        NaT
1    Batman  Batmobile 1940-04-25
2  Catwoman   Bullwhip        NaT
```

the example that uses the `subset` parameter,

```pycon
>>> df.dropna(subset=['name', 'born'])
       name        toy       born
1    Batman  Batmobile 1940-04-25
```

yields the same output as the example that doesn't pass in any arguments,

```pycon
>>> df.dropna()
     name        toy       born
1  Batman  Batmobile 1940-04-25
```

I think changing the `subset` example to

```pycon
>>> df.dropna(subset=['name', 'toy'])
       name        toy       born
1    Batman  Batmobile 1940-04-25
2  Catwoman   Bullwhip        NaT
```

would better illustrate the concept, as its result contains a row with one missing value, just in a column that wasn't specified in `subset`.

-----

I'm not able to test the docstring locally at the moment, but I ran the code in my console up to the new example, and it worked as expected.

```pycon
>>> import pandas as pd
>>> import numpy as np
>>> df = pd.DataFrame({"name": ['Alfred', 'Batman', 'Catwoman'],
...                    "toy": [np.nan, 'Batmobile', 'Bullwhip'],
...                    "born": [pd.NaT, pd.Timestamp("1940-04-25"),
...                             pd.NaT]})
>>> df
       name        toy       born
0    Alfred        NaN        NaT
1    Batman  Batmobile 1940-04-25
2  Catwoman   Bullwhip        NaT
>>> df.dropna()
     name        toy       born
1  Batman  Batmobile 1940-04-25
>>> df.dropna(axis='columns')
       name
0    Alfred
1    Batman
2  Catwoman
>>> df.dropna(how='all')
       name        toy       born
0    Alfred        NaN        NaT
1    Batman  Batmobile 1940-04-25
2  Catwoman   Bullwhip        NaT
>>> df.dropna(thresh=2)
       name        toy       born
1    Batman  Batmobile 1940-04-25
2  Catwoman   Bullwhip        NaT
>>> df.dropna(subset=['name', 'toy'])
       name        toy       born
1    Batman  Batmobile 1940-04-25
2  Catwoman   Bullwhip        NaT
```
Thanks @tywmick 