Update docs for custom attributes to add annotations

[robrap]

Use of custom attributes is documented here:
https://edx.readthedocs.io/projects/edx-django-utils/en/latest/monitoring/how_tos/using_custom_attributes.html

We've been annotating custom attributes as seen in these examples:
https://github.com/search?q=(org%3Aedx%20OR%20org%3Aopenedx)%20custom_attribute_name&type=codev

    # .. custom_attribute_name: jwt_auth_check_symmetric_key
    # .. custom_attribute_description: True if symmetric keys will also be used for checking
    #   the JWT signature, and False if only asymmetric keys will be used.
    set_custom_attribute('jwt_auth_check_symmetric_key', decode_symmetric_token)

1. The docs should detail this so we can more easily point people to this.
2. As part of this or a separate issue, we should add Sphinx documentation for these annotations like was done for toggles and settings in https://github.com/openedx/code-annotations/tree/master/code_annotations/contrib/sphinx/extensions