text: Title: [DOCS] Encounter ValueError When Following User Guide Annotation Label Schema
  Body: ### URL(s) with the issue
  
  [User Guide: annotation label schema](https://docs.voxel51.com/user_guide/annotation.html#label-schema)
  
  ### Description of proposal (what needs changing)
  
  **Description of Error**
  I encountered following error when I use the schema provided in user guide
  `ValueError: Attribute 'attr3' of type 'text' requires a list of values`
  
  **Potential Fix**
  I resolved the error by adding in `"values":[]` after line 24. But I'm not sure if this is the best approach to address this problem.
  
  The code example would be look like:
  ```
  anno_key = "..."
  
  label_schema = {
      "new_field": {
          "type": "classifications",
          "classes": ["class1", "class2"],
          "attributes": {
              "attr1": {
                  "type": "select",
                  "values": ["val1", "val2"],
                  "default": "val1",
              },
              "attr2": {
                  "type": "radio",
                  "values": [True, False],
                  "default": False,
              }
          },
      },
      "existing_field": {
          "classes": ["class3", "class4"],
          "attributes": {
              "attr3": {
                  "type": "text",
                  "values":[]
              }
          }
      },
  }
  
  dataset.annotate(anno_key, label_schema=label_schema)
  ```
  
  **Other questions**
  I'm using label schema to construct annotation task in CVAT. My understanding is that by including `existing_field`, I could modify existing annotation on CVAT. However, when I do so I got:
  `A field with label type 'detections' is already being annotated. Ignoring field 'ground_truth'...`
  (ground_truth here is my `existing_field`)
  and the `existing_field` is not loaded to CVAT. Can someone enlighten me where I made a mistake or direct me to relevant docs. Thanks in advance
  
  ### Willingness to contribute
  
  The FiftyOne Community encourages documentation contributions. Would you or another member of your organization be willing to contribute a fix for this documentation issue to the FiftyOne codebase?
  
  -   [ ] Yes. I can contribute a documentation fix independently
  -   [x] Yes. I would be willing to contribute a documentation fix with guidance from the FiftyOne community
  -   [ ] No. I cannot contribute a documentation fix at this time
  