Update "High Availability" docs to discuss retryable reads and writes

Update "High Availability" docs to discuss retryable reads and writes: https://pymongo.readthedocs.io/en/stable/examples/high_availability.html

The current page has some outdated and at best misleading info. We should discuss retryable reads/writes, the benefits those features bring, the limitations (only 1 retry), and how to write highly available apps in modern pymongo.