Add microphone sound_level_db doc

Author: microbit-grace

lang/en/typeshed/stdlib/microbit/microphone.pyi

@@ -71,14 +71,23 @@ def set_threshold(event: SoundEvent, value: int) -> None:
     ...
 
 def sound_level() -> int:
-    """Get the sound pressure level.
+    """Get the sound pressure level in the range 0 to 255.
 
     Example: ``microphone.sound_level()``
 
     :return: A representation of the sound pressure level in the range 0 to 255.
     """
     ...
 
+def sound_level_db() -> int:
+    """Get the sound pressure level in decibels.
+
+    Example: ``microphone.sound_level_db()``
+
+    :return: A representation of the sound pressure level in decibels (dB).
+    """
+    ...
+
 def record(duration: int, rate: int = 7812) -> AudioFrame:
     """Record sound into an ``AudioFrame`` for the amount of time indicated by
     ``duration`` at the sampling rate indicated by ``rate``.