DOC: document default behaviour of sample methods when random_state = None (#59073)

* Document the default None random_state of sample

Previously the default random state of the Series/DataFrame/GroupBy
`sample` methods was unclear as the documentation for the behaviour
when `random_state=None` was hidden in the
`pandas.core.common.random_state` helper function.

This commit documents how `sample` will just defer to the random state
of `numpy.random` when no `random_state` parameter is passed.

* Add Series.sample to GroupBy See Also docstring

* Format None type as code in doc strings

Author: alexmalins

pandas/core/generic.py

@@ -5731,7 +5731,7 @@ def sample(
         replace : bool, default False
             Allow or disallow sampling of the same row more than once.
         weights : str or ndarray-like, optional
-            Default 'None' results in equal probability weighting.
+            Default ``None`` results in equal probability weighting.
             If passed a Series, will align with target object on index. Index
             values in weights not found in sampled object will be ignored and
             index values in sampled object not in weights will be assigned
@@ -5746,6 +5746,7 @@ def sample(
         random_state : int, array-like, BitGenerator, np.random.RandomState, np.random.Generator, optional
             If int, array-like, or BitGenerator, seed for random number generator.
             If np.random.RandomState or np.random.Generator, use as given.
+            Default ``None`` results in sampling with the current state of np.random.
 
             .. versionchanged:: 1.4.0
 

pandas/core/groupby/groupby.py

@@ -5389,6 +5389,7 @@ def sample(
         random_state : int, array-like, BitGenerator, np.random.RandomState, np.random.Generator, optional
             If int, array-like, or BitGenerator, seed for random number generator.
             If np.random.RandomState or np.random.Generator, use as given.
+            Default ``None`` results in sampling with the current state of np.random.
 
             .. versionchanged:: 1.4.0
 
@@ -5403,6 +5404,7 @@ def sample(
         See Also
         --------
         DataFrame.sample: Generate random samples from a DataFrame object.
+        Series.sample: Generate random samples from a Series object.
         numpy.random.choice: Generate a random sample from a given 1-D numpy
             array.