Add deprecation notice for future synthio.BlockBiquad support (adaf…

…ruit#9756).

shared-bindings/audiofilters/Filter.c

@@ -156,7 +156,9 @@ MP_PROPERTY_GETSET(audiofilters_filter_filter_obj,
 //|     def low_pass_filter(
 //|         self, frequency: float, q_factor: float = 0.7071067811865475
 //|     ) -> synthio.Biquad:
-//|         """Construct a low-pass filter with the given parameters and update the `filter` property.
+//|         """Construct a low-pass filter with the given parameters and update the `filter` property. **DEPRECATED**
+//|
+//|         Instead, construct a `synthio.BlockBiquad` directly.
 //|
 //|         ``frequency``, called f0 in the cookbook, is the corner frequency in Hz
 //|         of the filter.
@@ -197,7 +199,9 @@ MP_DEFINE_CONST_FUN_OBJ_KW(audiofilters_filter_lpf_obj, 1, audiofilters_filter_o
 //|     def high_pass_filter(
 //|         self, frequency: float, q_factor: float = 0.7071067811865475
 //|     ) -> synthio.Biquad:
-//|         """Construct a high-pass filter with the given parameters and update the `filter` property.
+//|         """Construct a high-pass filter with the given parameters and update the `filter` property. **DEPRECATED**
+//|
+//|         Instead, construct a `synthio.BlockBiquad` directly.
 //|
 //|         ``frequency``, called f0 in the cookbook, is the corner frequency in Hz
 //|         of the filter.
@@ -227,7 +231,9 @@ MP_DEFINE_CONST_FUN_OBJ_KW(audiofilters_filter_hpf_obj, 1, audiofilters_filter_o
 //|     def band_pass_filter(
 //|         self, frequency: float, q_factor: float = 0.7071067811865475
 //|     ) -> synthio.Biquad:
-//|         """Construct a band-pass filter with the given parameters and update the `filter` property.
+//|         """Construct a band-pass filter with the given parameters and update the `filter` property. **DEPRECATED**
+//|
+//|         Instead, construct a `synthio.BlockBiquad` directly.
 //|
 //|         ``frequency``, called f0 in the cookbook, is the center frequency in Hz
 //|         of the filter.