[docs] Add guide to avoid logs from the default logger (#195)

Fixes https://github.com/apache/pulsar-client-python/issues/194

### Motivation

When the Python logger is configured as the logger, during the exit
process of a Python script, the default logger will be used for logging
instead because the Python logger is unavailable after the interpreter
is finalized. It might lead to some unexpected outputs to standard
output.

pulsar/__init__.py

diff --git a/pulsar/__init__.py b/pulsar/__init__.py
index 29bb034..99b225f 100644
--- a/pulsar/__init__.py
+++ b/pulsar/__init__.py
@@ -532,6 +532,19 @@
             matches the common name on the TLS certificate presented by the endpoint.
         logger: optional
             Set a Python logger for this Pulsar client. Should be an instance of `logging.Logger`.
+            It should be noted that if the Python logger is configured, during the termination of the Python
+            interpreter, the Python logger will be unavailable and the default logger will be used for logging.
+            To avoid strange behavior, you'd better delete all instances explicitly before exiting.
+
+            .. code-block:: python
+
+                import logging
+                client = Client(service_url, logger=logging.getLogger('pulsar'))
+                producer = client.create_producer(topic)
+                # ...
+                del producer
+                del client
+
         connection_timeout_ms: int, default=10000
             Set timeout in milliseconds on TCP connections.
         listener_name: str, optional