Added figsize for autocorrelation_plot and lag_plot (#31650)
- closes #31650
- Passed pandas.tests.plotting.test_misc.py
- Now creating new axes with "fig, ax = plt.subplots(1, 1, figsize=figsize)"
- Also changed plotting.lag_plot regarding the same issue
- Figure size is only changed if no axis parameter is given


@jqmviegas do we have tests that cover all of the changed code?  codecov seems to think not > @jqmviegas do we have tests that cover all of the changed code? codecov seems to think not

@jbrockmendel  Added checks using the _check_plot_works(...) with the figsize keyword but I'm not sure if that does the trick. Unsure how to improve testing regarding this issue. Running coverage.py indicates that everything is covered in the methods I worked on. @jbrockmendel I'm almost sure that this is the result of the CI running the fast tests. The tests for the methods I worked on are decorated with @pytest.mark.slow. I was able to reproduce the results of codecov/patch running the following:

`pytest --cov=pandas --skip-slow pandas/tests/plotting/test_misc.py`

This returns:

```
Name                                        Stmts   Miss  Cover   Missing
-------------------------------------------------------------------------
...
pandas/plotting/_matplotlib/misc.py           269    167    38%   63-69, 113, 118-196, 202-256, 261-303, 332, 340-342, 344-348, 388-402, 406-437
```

Running the full tests with:

`pytest --cov=pandas pandas/tests/plotting/test_misc.py`

Returns:


```
Name                                        Stmts   Miss  Cover   Missing
-------------------------------------------------------------------------
...
pandas/plotting/_matplotlib/misc.py           269     52    81%   63-69, 113, 187, 202-256, 332, 340-342, 344-348
```
No missing lines include the functions autocorrelation_plot or lag_plot.  @charlesdong1991 can you take a look; i think you know the plotting code better than i do @jbrockmendel , @charlesdong1991 - Changed the tests to ensure `figsize` changes to what we set. Additionally improved the docstrings and added change to 1.1 rst. CI failing due to other issues I think. can you merge master and update to comments closing as stale if u want to continue pls ping 