Updated notes on using a cache instance when initializing OAuth.

[iSOLveIT]

DO NOT SEND ANY SECURITY FIX HERE. Please read the "Security Reporting" section
on README.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Other, please describe: Updated Flask OAuth Client documentation.

---

• You consent that the copyright of your pull request source code belongs to Authlib's author.

The Flask OAuth Client documentation excludes important information about creating a cache instance for OAuth initialization.

Under FlaskClient framework_integration.py source code,

# File: authlib/integrations/base_client/framework_integration.py

def clear_state_data(self, session, state):
        key = f'_state_{self.name}_{state}'
        if self.cache:
            self.cache.delete(key)   # Calls the delete method on the cache object
        else:
            ...

You are required to have a delete method for the cache object else you get an AttributeError like this AttributeError("'Cache' object has no attribute 'delete'").

This PR updates the documentation to inform users about the need to have a delete method as part of their cache instance.

[lepture]

Thanks