improve doc about sampling configuration (#406)

Author: srprash

docs/configurations.rst

@@ -16,13 +16,18 @@ To turn off sampling, use code like the following::
     from aws_xray_sdk.core import xray_recorder
     xray_recorder.configure(sampling=False)
 
-You can also configure the sampling rules::
+By default, the SDK uses sampling rules configured in your AWS account. You can also configure the backup sampling rules locally::
 
     xray_recorder.configure(sampling_rules=your_rules)
 
 The input can either be an absolute path to your sampling rule
 *.json* file or a dictionary.
 
+To use only local rules for sampling, configure the recorder with a ``LocalSampler``::
+
+    from aws_xray_sdk.core.sampling.local.sampler import LocalSampler
+    xray_recorder.configure(sampler=LocalSampler())
+
 The following code is an example of a rule configuration::
 
     {