Further improve backend tutorial.

[anntzer]

1. Don't do use("ps") (in practice one never needs to set that backend
   manually as we autoswitch to "ps" when saving a postscript); instead
   have use("qt5agg") as example.
2. The requirement to call use() before importing pyplot is obsolete.
   Actually even "use() needs to be called before creating any figure"
   is a bit too restrictive because switching to noninteractive backends
   is always possible... but no need to go into this in the intro
   tutorial.
3. Reword description of "default" backend selection.

followup to #14549.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way