adding ntoes about bearer tokens and authentication to credential handling section

Author: Aaron Gonzales

README.rst

@@ -56,8 +56,15 @@ fields; Enterprise clients require ``username``, ``password``, and
 ``endpoint``. If you do not specify the ``account_type``, we attempt to
 discern the account type and declare a warning about this behavior.
 
+For premium search products, we are using app-only authentication and
+the bearer tokens are not delivered with an expiration time. They can be
+invalidated. Please see
+`here <https://developer.twitter.com/en/docs/basics/authentication/overview/application-only>`__
+for an overview of the premium authentication method.
+
 We support both YAML-file based methods and environment variables for
-access, and provide flexible handling with sensible defaults.
+storing credentials, and provide flexible handling with sensible
+defaults.
 
 YAML method
 -----------

examples/credential_handling.ipynb

@@ -11,7 +11,9 @@
     "Premium clients will require the `bearer_token` and `endpoint` fields; Enterprise clients require `username`, `password`, and `endpoint`.\n",
     "If you do not specify the `account_type`, we attempt to discern the account type and declare a warning about this behavior.\n",
     "\n",
-    "We support both YAML-file based methods and environment variables for access, and provide flexible handling with sensible defaults.\n",
+    "For premium search products, we are using app-only authentication and the bearer tokens are not delivered with an expiration time. They can be invalidated. Please see [here](https://developer.twitter.com/en/docs/basics/authentication/overview/application-only) for an overview of the premium authentication method.\n",
+    "\n",
+    "We support both YAML-file based methods and environment variables for storing credentials, and provide flexible handling with sensible defaults.\n",
     "\n",
     "\n",
     "## YAML method\n",

examples/credential_handling.rst

@@ -11,8 +11,15 @@ fields; Enterprise clients require ``username``, ``password``, and
 ``endpoint``. If you do not specify the ``account_type``, we attempt to
 discern the account type and declare a warning about this behavior.
 
+For premium search products, we are using app-only authentication and
+the bearer tokens are not delivered with an expiration time. They can be
+invalidated. Please see
+`here <https://developer.twitter.com/en/docs/basics/authentication/overview/application-only>`__
+for an overview of the premium authentication method.
+
 We support both YAML-file based methods and environment variables for
-access, and provide flexible handling with sensible defaults.
+storing credentials, and provide flexible handling with sensible
+defaults.
 
 YAML method
 -----------