PYTHON-3445 Improve documentation for custom readPreference tags (#1068)

Author: juliusgeo

README.rst

@@ -25,7 +25,7 @@ For issues with, questions about, or feedback for PyMongo, please look into
 our `support channels <https://support.mongodb.com/welcome>`_. Please
 do not email any of the PyMongo developers directly with issues or
 questions - you're more likely to get an answer on the `MongoDB Community
-Forums <https://developer.mongodb.com/community/forums/tag/python-driver>`_.
+Forums <https://www.mongodb.com/community/forums/tag/python>`_.
 
 Bugs / Feature Requests
 =======================

RELEASE.rst

@@ -84,7 +84,7 @@ Doing a Release
 13. Publish the release version in Jira.
 
 14. Announce the release on:
-    https://developer.mongodb.com/community/forums/c/community/release-notes/
+    https://www.mongodb.com/community/forums/c/announcements/driver-releases/110
 
 15. File a ticket for DOCSP highlighting changes in server version and Python
     version compatibility or the lack thereof, for example:

pymongo/read_preferences.py

@@ -151,6 +151,13 @@ def tag_sets(self) -> _TagSets:
         set, ``{}``, means "read from any member that matches the mode,
         ignoring tags." MongoClient tries each set of tags in turn
         until it finds a set of tags with at least one matching member.
+        For example, to only send a query to an analytic node::
+
+           Nearest(tag_sets=[{"node":"analytics"}])
+
+        Or using :class:`SecondaryPreferred`::
+
+           SecondaryPreferred(tag_sets=[{"node":"analytics"}])
 
            .. seealso:: `Data-Center Awareness
                <https://www.mongodb.com/docs/manual/data-center-awareness/>`_
@@ -518,7 +525,11 @@ def make_read_preference(
 
 
 class ReadPreference(object):
-    """An enum that defines the read preference modes supported by PyMongo.
+    """An enum that defines some commonly used read preference modes.
+
+    Apps can also create a custom read preference, for example::
+
+       Nearest(tag_sets=[{"node":"analytics"}])
 
     See :doc:`/examples/high_availability` for code examples.