Add info about default topics prefixes to the docs (#662)

docs/connectors/sources/README.md

@@ -37,12 +37,19 @@ For good performance, each source runs in a subprocess. Quix Streams automatical
 
 For multiplatform support, Quix Streams starts the source process using the [spawn](https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods) approach. As a side effect, each Source instance must be pickleable. If a source needs to handle unpickleable objects, it's best to initialize those in the source subprocess (in the `BaseSource.start` or `Source.run` methods).  
 
-## Customize Topic Configuration
+## Topics
 
 Sources work by sending data to intermediate Kafka topics, which StreamingDataFrames then consume and process.
 
-By default, each Source provides a default topic based on its configuration.  
-To customize the topic config, pass a new `Topic` object to the `app.dataframe()` method together with the Source instance. 
+By default, each Source provides a default topic based on its configuration by implementing the `default_topic()` method.
+
+!!! warning "New in 3.4.0" 
+
+    Since v3.4.0, the default topics names are always prefixed with `"source__"`.
+
+
+To customize the topic name or configuration, pass a new `Topic` object to the `app.dataframe()` method together with the Source instance.
+
 
 **Example:**
 
@@ -109,7 +116,6 @@ To customize the topic the Source will use, create a new `Topic` and pass it to
 
 ```python
 from quixstreams import Application
-from quixstreams.sources import CSVSource
 from quixstreams.models.topics import TopicConfig
 
 def main():

quixstreams/app.py

@@ -676,7 +676,8 @@ def add_source(self, source: BaseSource, topic: Optional[Topic] = None) -> Topic
 
         :param source: a :class:`quixstreams.sources.BaseSource` instance
         :param topic: the :class:`quixstreams.models.Topic` instance the source will produce to
-            Default: the source default
+            Default - the topic generated by the `source.default_topic()` method.
+            Note: the names of default topics are prefixed with "source__".
         """
         if not topic:
             # Prefix the name of the default topic generated by the Source

quixstreams/sources/base/source.py

@@ -129,6 +129,8 @@ def default_topic(self) -> Topic:
         This method is triggered when the topic is not provided to the source.
 
         The source must return a default topic configuration.
+
+        Note: if the default topic is used, the Application will prefix its name with "source__".
         """
 
 
@@ -311,6 +313,8 @@ def default_topic(self) -> Topic:
         Return a default topic matching the source name.
         The default topic will not be used if the topic has already been provided to the source.
 
+        Note: if the default topic is used, the Application will prefix its name with "source__".
+
         :return: `quixstreams.models.topics.Topic`
         """
         return Topic(