docs: add deprecation to .nullifzero docstring

Although users got a deprecationWarning
whenever they used these in code,
in the docs there is no
indication they are deprecated.

ibis/expr/types/numeric.py

@@ -8,10 +8,10 @@
 
 import ibis
 import ibis.expr.operations as ops
+from ibis import util
 from ibis.common.exceptions import IbisTypeError
 from ibis.expr.types.core import _binop
 from ibis.expr.types.generic import Column, Scalar, Value
-from ibis import util
 
 if TYPE_CHECKING:
     import ibis.expr.types as ir
@@ -483,12 +483,12 @@ def sqrt(self) -> NumericValue:
 
     @util.deprecated(instead="use nullif(0)", as_of="7.0", removed_in="8.0")
     def nullifzero(self) -> NumericValue:
-        """Return `NULL` if an expression is zero."""
+        """DEPRECATED: Use `nullif(0)` instead."""
         return self.nullif(0)
 
     @util.deprecated(instead="use fillna(0)", as_of="7.0", removed_in="8.0")
     def zeroifnull(self) -> NumericValue:
-        """Return zero if an expression is `NULL`."""
+        """DEPRECATED: Use `fillna(0)` instead."""
         return self.fillna(0)
 
     def acos(self) -> NumericValue: