DOC: document convention argument for resample() (#16965)

* DOC: document convention argument for resample()

pandas/core/generic.py

@@ -4826,6 +4826,8 @@ def resample(self, rule, how=None, axis=0, fill_method=None, closed=None,
         label : {'right', 'left'}
             Which bin edge label to label bucket with
         convention : {'start', 'end', 's', 'e'}
+            For PeriodIndex only, controls whether to use the start or end of
+            `rule`
         loffset : timedelta
             Adjust the resampled time labels
         base : int, default 0
@@ -4946,6 +4948,47 @@ def resample(self, rule, how=None, axis=0, fill_method=None, closed=None,
         2000-01-01 00:06:00    26
         Freq: 3T, dtype: int64
 
+        For a Series with a PeriodIndex, the keyword `convention` can be
+        used to control whether to use the start or end of `rule`.
+
+        >>> s = pd.Series([1, 2], index=pd.period_range('2012-01-01',
+                                                        freq='A',
+                                                        periods=2))
+        >>> s
+        2012    1
+        2013    2
+        Freq: A-DEC, dtype: int64
+
+        Resample by month using 'start' `convention`. Values are assigned to
+        the first month of the period.
+
+        >>> s.resample('M', convention='start').asfreq().head()
+        2012-01    1.0
+        2012-02    NaN
+        2012-03    NaN
+        2012-04    NaN
+        2012-05    NaN
+        Freq: M, dtype: float64
+
+        Resample by month using 'end' `convention`. Values are assigned to
+        the last month of the period.
+
+        >>> s.resample('M', convention='end').asfreq()
+        2012-12    1.0
+        2013-01    NaN
+        2013-02    NaN
+        2013-03    NaN
+        2013-04    NaN
+        2013-05    NaN
+        2013-06    NaN
+        2013-07    NaN
+        2013-08    NaN
+        2013-09    NaN
+        2013-10    NaN
+        2013-11    NaN
+        2013-12    2.0
+        Freq: M, dtype: float64
+
         For DataFrame objects, the keyword ``on`` can be used to specify the
         column instead of the index for resampling.