[Cleanup] Pass reason for bulk message deletion to audit log (#5863)

Co-authored-by: Jakub Kuczys <me@jacken.men>

redbot/core/utils/mod.py

@@ -1,6 +1,6 @@
 import asyncio
 from datetime import timedelta
-from typing import List, Iterable, Union, TYPE_CHECKING, Dict
+from typing import List, Iterable, Union, TYPE_CHECKING, Dict, Optional
 
 import discord
 
@@ -12,6 +12,8 @@ if TYPE_CHECKING:
 async def mass_purge(
     messages: List[discord.Message],
     channel: Union[discord.TextChannel, discord.VoiceChannel, discord.Thread],
+    *,
+    reason: Optional[str] = None,
 ):
     """Bulk delete messages from a channel.
 
@@ -29,6 +31,8 @@ async def mass_purge(
         The messages to bulk delete.
     channel : `discord.TextChannel`, `discord.VoiceChannel`, or `discord.Thread`
         The channel to delete messages from.
+    reason : `str`, optional
+        The reason for bulk deletion, which will appear in the audit log.
 
     Raises
     ------
@@ -43,7 +47,7 @@ async def mass_purge(
         # discord.NotFound can be raised when `len(messages) == 1` and the message does not exist.
         # As a result of this obscure behavior, this error needs to be caught just in case.
         try:
-            await channel.delete_messages(messages[:100])
+            await channel.delete_messages(messages[:100], reason=reason)
         except discord.errors.HTTPException:
             pass
         messages = messages[100:]