Document that User.all_channels property is read only

Background/Problem
If the user attempts to start Sync Gateway with Users.all_channels property set in the Database Configuration. For Example the following is set in Database Configuration:

"users":{

        "GUEST": {"admin_channels": ["*"], "all_channels": ["*"], "disabled": true}

      }

User gets the following error which is expected as all_channels field is read only in version 3.x and should only be populated by Sync Gateway:

2024-08-05T22:39:33.464Z [ERR] Couldn't start Sync Gateway: 400 all_channels is read-only -- base.FatalfCtx() at logging.go:148

In a customer case we discovered that the user was unclear about this behavior by reading the documentation for Users.all_channels property i.e. they were unable to understand that the field is read only and why the field is read only.

Request
Please update the documentation for Users.all_channels property to include that this field is read only and better explain why this field is read only.