Mention source of `elements` database in docstring

[ericpre]

As mentioned in https://github.com/hyperspy/exspy/discussions/85#discussioncomment-10678716.

Progress of the PR

• [x] Mention source of elements database in docstring,
• [x] docstring updated (if appropriate),
• [x] update user guide (if appropriate),
• [n/a] added tests,
• [x] add a changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• [x] Check formatting of the changelog entry (and eventual user guide changes) in the docs/readthedocs.org:exspy build of this PR (link in github checks)
• [x] ready for review.

[codecov[bot]]

Codecov Report

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

Project coverage is 88.70%. Comparing base (ee31d0a) to head (0f9d7c0). Report is 5 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #87 +/- ## ======================================= Coverage 88.70% 88.70% ======================================= Files 77 77 Lines 7453 7453 Branches 1200 1200 ======================================= Hits 6611 6611 Misses 572 572 Partials 270 270 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[jlaehne]

Chantler reference is already mentioned in other docstrings, but way of mentioning is quite inconsistent.

[ericpre]

Chantler reference is already mentioned in other docstrings, but way of mentioning is quite inconsistent.

@jlaehne, is it better now with the references?

[jlaehne]

Chantler reference is already mentioned in other docstrings, but way of mentioning is quite inconsistent.

@jlaehne, is it better now with the references?

Maybe align also the citation in the Bibliography of the User Guide (use DOI, it currently uses the subtitle and not the title of the page).