DOC: Clarify Coords Report Example

[StefRe]

• add remark that this only pertains to the interactive backend
• PEP8 fixes

PR Summary

The coord reports example prominently shows the plot produced by the example. This can be a bit midleading as the demonstrated feature is only visible in the interactive backend. As this can lead to some confusion (see e.g. #2205), I added a short explanatory remark.
(the same pertains to the modifying the coordinate formatter example)

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation
[N/A] New features are documented, with examples if plot related.
[N/A] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
[N/A] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

• Documentation is sphinx and numpydoc compliant (the docs should build without error).

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[timhoffm]

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