bpo-28851: DOC: Retrofit changes to collections.rst from 2.7

[csabella]

Notes from the original change in 2.7:

Minor doc clean-up.

• Show list of fields option before showing the single string alternative.
• Remove the PS2 prompts so that the examples become cut-and-pastable.

[mention-bot]

@csabella, thanks for your PR! By analyzing the history of the files in this pull request, we identified @birkenfeld, @rhettinger and @tiran to be potential reviewers.

[vadmium]

This changes things unrelated to field_names, some of which were already made in revision 15aded8 and recently undone: https://bugs.python.org/issue27205. Did you try running the doc tests? If we cannot agree on the best way to mark up example code, I suggest to leave it as it is. Otherwise, it would be good to negotiate with the doc test bug or the people involved: @JelleZijlstra, @marco-buttu.

[csabella]

I ran './python -m test' and test_collections passed, but maybe that's not the test you mean? The bug ticket for 28851 didn't mention the other issue, so I thought it just needed to be retrofitted from that older revision into the newest docs.

[JelleZijlstra]

I think Martin was referring to doctests, which you can run by running make doctest in the Doc/ subdirectory.

[JelleZijlstra]

Otherwise I'm not familiar with the linked changes, but it seems better to drop unrelated modifications from this PR.

[serhiy-storchaka]

Please restore the PS2 prompts. The examples are cut-and-pastable, just press on the button at right top corner of an example box.

[csabella]

OK. I copied and pasted the comments from the original PR by RH, so he had made the bullet about them not being able to be cut and pasted. But, thank you for clarifying for me. This is all a learning experience and I appreciate everyone's patience. And thank you for telling me about 'make doctest'.