Using Literal["a"] instead of str in generic.py (#52742)

fixes: specify limit_area in interpolate more precisely
an specify how in asfreq more precisely

Author: kevx82

pandas/core/generic.py

@@ -7524,7 +7524,7 @@ def interpolate(
         limit: int | None = None,
         inplace: bool_t = False,
         limit_direction: Literal["forward", "backward", "both"] | None = None,
-        limit_area: str | None = None,
+        limit_area: Literal["inside", "outside"] | None = None,
         downcast: str | None = None,
         **kwargs,
     ) -> Self | None:
@@ -8346,7 +8346,7 @@ def asfreq(
         self,
         freq: Frequency,
         method: FillnaOptions | None = None,
-        how: str | None = None,
+        how: Literal["start", "end"] | None = None,
         normalize: bool_t = False,
         fill_value: Hashable = None,
     ) -> Self: