DOC: Add comments for the single level groupby

Author: Licht-T

pandas/core/groupby.py

@@ -2586,14 +2586,17 @@ def _get_grouper(obj, key=None, axis=0, level=None, sort=True,
     """
     group_axis = obj._get_axis(axis)
 
-    # validate that the passed level is compatible with the passed
+    # validate that the passed single level is compatible with the passed
     # axis of the object
     if level is not None:
+        # TODO: These two conditions are almost same.
+        # We should combine two.
         if isinstance(group_axis, MultiIndex):
             if is_list_like(level) and len(level) == 1:
                 level = level[0]
 
             if key is None and is_scalar(level):
+                # Get the level values from group_axis
                 key = group_axis.get_level_values(level)
                 level = None
 
@@ -2619,6 +2622,8 @@ def _get_grouper(obj, key=None, axis=0, level=None, sort=True,
                 raise ValueError('level > 0 or level < -1 only valid with '
                                  ' MultiIndex')
 
+            # NOTE: `group_axis` and `group_axis.get_level_values(level)`
+            # are same in this section.
             level = None
             key = group_axis