Update tooltip text for plot ensemble selector

[andreas-el]

Minor clarification on functionality.

User request:

When hoovering one get some info saying one can reorder, but it would be nice to extend the text to emphasize that it is done "by drag and drop".

• [x] PR title captures the intent of the changes, and is fitting for release notes.
• [x] Added appropriate release note label
• [x] Commit history is consistent and clean, in line with the contribution guidelines.
• [ ] Make sure tests pass locally (after every commit!)

When applicable

• [ ] When there are user facing changes: Updated documentation
• [ ] New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• [ ] Large PR: Prepare changes in small commits for more convenient review
• [ ] Bug fix: Add regression test for the bug
• [ ] Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.03%. Comparing base (48ab658) to head (352501e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #8119 +/- ## ======================================= Coverage 86.03% 86.03% ======================================= Files 383 383 Lines 23739 23739 Branches 619 628 +9 ======================================= Hits 20425 20425 - Misses 3233 3241 +8 + Partials 81 73 -8 ```

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