Incorrect tooltip content for model selection options

[thao-do]

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