Partial update docs

examples/own_filter.py

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+from aiogram import Router
+from aiogram.filters import Filter
+from aiogram.types import Message
+
+router = Router()
+
+
+class MyFilter(Filter):
+    def __init__(self, my_text: str) -> None:
+        self.my_text = my_text
+
+    async def __call__(self, message: Message) -> bool:
+        return message.text == self.my_text
+
+
+@router.message(MyFilter("hello"))
+async def my_handler(message: Message):
+    ...