DOC: Add behavior for Index argument in DataFrame.loc (#38109)

Author: hongshaoyang

pandas/core/indexing.py

@@ -259,10 +259,11 @@ def loc(self) -> "_LocIndexer":
           e.g. ``[True, False, True]``.
         - An alignable boolean Series. The index of the key will be aligned before
           masking.
+        - An alignable Index. The Index of the returned selection will be the input.
         - A ``callable`` function with one argument (the calling Series or
           DataFrame) and that returns valid output for indexing (one of the above)
 
-        See more at :ref:`Selection by Label <indexing.label>`
+        See more at :ref:`Selection by Label <indexing.label>`.
 
         Raises
         ------
@@ -332,6 +333,14 @@ def loc(self) -> "_LocIndexer":
                     max_speed  shield
         sidewinder          7       8
 
+        Index (same behavior as ``df.reindex``)
+
+        >>> df.loc[pd.Index(["cobra", "viper"], name="foo")]
+               max_speed  shield
+        foo
+        cobra          1       2
+        viper          4       5
+
         Conditional that returns a boolean Series
 
         >>> df.loc[df['shield'] > 6]