DOC: fix doctests for pandas/core/generic.py for new string dtype (#61907)

Author: arthurlw

pandas/core/generic.py

@@ -1918,7 +1918,7 @@ def keys(self) -> Index:
         b  2  4
         c  3  8
         >>> d.keys()
-        Index(['A', 'B'], dtype='object')
+        Index(['A', 'B'], dtype='str')
         """
         return self._info_axis
 
@@ -6276,7 +6276,7 @@ def dtypes(self):
         float              float64
         int                  int64
         datetime    datetime64[s]
-        string              object
+        string              str
         dtype: object
         """
         data = self._mgr.get_dtypes()
@@ -6838,7 +6838,7 @@ def convert_dtypes(
         0      a
         1      b
         2    NaN
-        dtype: object
+        dtype: str
 
         Obtain a Series with dtype ``StringDtype``.
 
@@ -8138,7 +8138,7 @@ def isna(self) -> Self:
         ... )
         >>> df
            age       born    name        toy
-        0  5.0        NaT  Alfred       None
+        0  5.0        NaT  Alfred        NaN
         1  6.0 1939-05-27  Batman  Batmobile
         2  NaN 1940-04-25              Joker
 
@@ -8211,7 +8211,7 @@ def notna(self) -> Self:
         ... )
         >>> df
            age       born    name        toy
-        0  5.0        NaT  Alfred       None
+        0  5.0        NaT  Alfred        NaN
         1  6.0 1939-05-27  Batman  Batmobile
         2  NaN 1940-04-25              Joker
 
@@ -10401,7 +10401,7 @@ def truncate(
         2    b
         3    c
         4    d
-        Name: A, dtype: object
+        Name: A, dtype: str
 
         The index values in ``truncate`` can be datetimes or string
         dates.