DOC: Add deprecated markers for downcast keyword (#56891)

* DOC: Add deprecated markers for downcast keyword

* Update docs

Author: phofl

pandas/core/generic.py

@@ -7208,6 +7208,8 @@ def fillna(
             or the string 'infer' which will try to downcast to an appropriate
             equal type (e.g. float64 to int64 if possible).
 
+            .. deprecated:: 2.2.0
+
         Returns
         -------
         {klass} or None
@@ -7543,6 +7545,8 @@ def ffill(
             or the string 'infer' which will try to downcast to an appropriate
             equal type (e.g. float64 to int64 if possible).
 
+            .. deprecated:: 2.2.0
+
         Returns
         -------
         {klass} or None
@@ -7734,6 +7738,8 @@ def bfill(
             or the string 'infer' which will try to downcast to an appropriate
             equal type (e.g. float64 to int64 if possible).
 
+            .. deprecated:: 2.2.0
+
         Returns
         -------
         {klass} or None