sync gateway 3.1 roles/users doesn't show collection properties in all places

This comes directly from https://issues.couchbase.com/browse/DOC-11062?focusedId=697241&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-697241

https://docs.couchbase.com/sync-gateway/current/roles.html should say:

user assigned to a Role is granted access to a channel, a part of collections

user assigned to a role is granted to access to a channel, which is associated with a specific collection

We should give an example of "How to create a role that is collection scoped:

curl -vX POST "http://localhost:4985/mydatabase/_roles/" -H

"accept: application/json" -H "Content-Type: application/json" -d

'{"name": "Edge1", "scopename": { "collectionname" : { "admin_channels": ["channel1", "channel3"]]} } } ' 

Since assigning admin_channels is collection scoped. The top level field of admin_channels will be ignored for anything that isn't a _default._default collection, and is provided for backward compatibility.

Additionally, on https://docs.couchbase.com/sync-gateway/current/roles.html this displays the following text, where tkn-db might be a typo?

Create a new role using the /{tkn-db}/_role/ endpoint.

All of these concepts apply to https://docs.couchbase.com/sync-gateway/current/users.html as well.

Additionally:

• https://docs.couchbase.com/sync-gateway/current/rest-api-admin.html#/Database_Security/put_db__role_name
• https://docs.couchbase.com/sync-gateway/current/rest-api-admin.html#/Database_Security/get_db__role_name
• https://docs.couchbase.com/sync-gateway/current/rest-api-admin.html#/Database_Security/post_db__user_
• https://docs.couchbase.com/sync-gateway/current/rest-api-admin.html#/Database_Security/get_db__user_

Need to show "scopename" and "collectionname" instead of "additionalProp1" and "additionalProp2"

We can fix some of the examples by providing `example:` in the openapi docs format, and that can be done in collaboration with SG team or docs team can modify our openAPI docs.