Incorrect tooltip content for model selection options

**Describe the bug**
Current tooltips for "Start a new model" and "Select an existing model" are incorrect

**To Reproduce**
Steps to reproduce the behavior:
1. On plugin's start screen, hover over the (?) of to see the tooltip

**Screenshots**

**Solution**
| Label | Current tooltip | New tooltip |
| ----- | ------|------|
| Start a new model | Use your own data to train a model from scratch | Use your own data to train a model from scratch or by using weight of a trained model |
| Select an existing model | Resume training from pretrained model weights | Use trained model to generation segmentation prediction