[BUG] DataFrame.describe has incorrect results for top in categorical column

[bdice]

Describe the bug
DataFrame.describe gives results for top in a categorical column that don't match pandas. The categories each occur once in this example, so the "top" is somewhat arbitrary, but we should match pandas' behavior if possible.

Reproduction & Expected Behavior
This is a minimal reproduction. Note that cudf gives f as the "top", while pandas and the cudf docstrings (see full reproduction) indicate that d should be the "top".

>>> cudf.Series(["d", "e", "f"], dtype="category").describe()
count     3
unique    3
top       f
freq      1
dtype: object
>>> pd.Series(["d", "e", "f"], dtype="category").describe()
count     3
unique    3
top       d
freq      1
dtype: object

The full error reproduction follows. 4 of 21 doctests (#9815) from the DataFrame.describe docstring fail because of this error.

Click to expand full error output

File "/home/bdice/code/cudf/python/cudf/cudf/core/series.py", line 3456, in Series.describe
Failed example:
    df.describe(include='all')
Expected:
           categorical numeric object
    count            3     3.0      3
    unique           3    <NA>      3
    top              d    <NA>      a
    freq             1    <NA>      1
    mean          <NA>     2.0   <NA>
    std           <NA>     1.0   <NA>
    min           <NA>     1.0   <NA>
    25%           <NA>     1.5   <NA>
    50%           <NA>     2.0   <NA>
    75%           <NA>     2.5   <NA>
    max           <NA>     3.0   <NA>
Got:
           categorical numeric object
    count            3     3.0      3
    unique           3    <NA>      3
    top              f    <NA>      a
    freq             1    <NA>      1
    mean          <NA>     2.0   <NA>
    std           <NA>     1.0   <NA>
    min           <NA>     1.0   <NA>
    25%           <NA>     1.5   <NA>
    50%           <NA>     2.0   <NA>
    75%           <NA>     2.5   <NA>
    max           <NA>     3.0   <NA>
**********************************************************************
File "/home/bdice/code/cudf/python/cudf/cudf/core/series.py", line 3508, in Series.describe
Failed example:
    df.describe(include=['category'])
Expected:
           categorical
    count            3
    unique           3
    top              d
    freq             1
Got:
           categorical
    count            3
    unique           3
    top              f
    freq             1
**********************************************************************
File "/home/bdice/code/cudf/python/cudf/cudf/core/series.py", line 3517, in Series.describe
Failed example:
    df.describe(exclude=[np.number])
Expected:
           categorical object
    count            3      3
    unique           3      3
    top              d      a
    freq             1      1
Got:
           categorical object
    count            3      3
    unique           3      3
    top              f      a
    freq             1      1
**********************************************************************
File "/home/bdice/code/cudf/python/cudf/cudf/core/series.py", line 3526, in Series.describe
Failed example:
    df.describe(exclude=[object])
Expected:
           categorical numeric
    count            3     3.0
    unique           3    <NA>
    top              d    <NA>
    freq             1    <NA>
    mean          <NA>     2.0
    std           <NA>     1.0
    min           <NA>     1.0
    25%           <NA>     1.5
    50%           <NA>     2.0
    75%           <NA>     2.5
    max           <NA>     3.0
Got:
           categorical numeric
    count            3     3.0
    unique           3    <NA>
    top              f    <NA>
    freq             1    <NA>
    mean          <NA>     2.0
    std           <NA>     1.0
    min           <NA>     1.0
    25%           <NA>     1.5
    50%           <NA>     2.0
    75%           <NA>     2.5
    max           <NA>     3.0
**********************************************************************