Backport PR #56891 on branch 2.2.x (DOC: Add deprecated markers for downcast keyword) (#56893)

Backport PR #56891: DOC: Add deprecated markers for downcast keyword

Co-authored-by: Patrick Hoefler <61934744+phofl@users.noreply.github.com>

Author: meeseeksmachine

pandas/core/generic.py

@@ -7187,6 +7187,8 @@ 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
@@ -7522,6 +7524,8 @@ 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
@@ -7713,6 +7717,8 @@ 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