DOC: describe any expected ordering of value counts when sort=False

[mvashishtha]

Pandas version checks

• I have checked that the issue still exists on the latest versions of the docs on main here

Location of the documentation

• https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.value_counts.html
• https://pandas.pydata.org/docs/reference/api/pandas.Series.value_counts.html

Documentation problem

The documentation says nothing about what value_counts() for series or dataframe do to order when sort=False. If there's no guarantee about the order, then the docs should say that. What I've observed is that dataframe value_counts sorts by values when sort=False, while dataframe value_counts sorts by the order that the keys first appeared in the data. For example:

import pandas as pd

df = pd.DataFrame(['b', 'a', 'a'])
# dataframe sorts by values, so index is [a, b]
print(df.value_counts(sort=False))
# dataframe sorts by order of row where each value first appears, so index is [b, a]
print(df[0].value_counts(sort=False))

Suggested fix for documentation

Explain what order to expect, if any, when sort=False.

[VomV]

take

[VomV]

@phofl I have made the changes but it's failing one of the tests (Ubuntu / Numpy Dev Test(not single_cpu)). This is my first contribution and I would really appreciate any help.

[phofl]

CI is failing right now on numpy dev, not related to your pr