Improve UX in column pinning both sides samples

Currently, the buttons in the samples simply say `Unpin Column`, `Pin Right`, and `Pin Left`. The button labels would be better to specify that they work for selected columns, i.e `Unpin Selected Columns`, `Pin Selected Columns Right`, and `Pin Selected Columns Left`.

This should be done for all platforms:
- [x] Angular
- [x] React
- [x] WC
- [x] Blazor