add pre-requisite explainer

Author: sbSteveK

awscrt/io.py

@@ -41,7 +41,8 @@ def init_logging(log_level, file_name):
 
 
 def set_log_level(log_level):
-    """Change the log level of `awscrt`.
+    """Change the log level of `awscrt`. init_logging() must have been called
+    before using this function.
 
     Args:
         log_level (LogLevel): Display messages of this importance and higher.