docs: add example for glab mr note command
Description
The following user feedback was provided on the glab mr note page:
A lot information is missing, i.e. how to ensure that there will be Newlines printed?
This MR adds usage examples to the glab mr note CLI command. The examples show three common ways to use the command:
- Adding a comment to a specific merge request by ID.
- adding a comment to the current branch's merge request
- Opening an editor for longer comments.