docs: Improve Container.pull docs to avoid leaky files

[tonyandrewmeyer]

Container.pull (and the Client.pull it wraps) return an opened file that is never explicitly closed. This leaks a resource until garbage collection (which then emits a warning), and in the case of encoding=None can leak that resource until process end. This is not a significant problem, but we should document the clean, rather than leaky, way to use these methods.

The how-to guide has poor suggestions as well, but those are all being replaced in #2470 (to use pathops) so are not worth fixing here.

Fixes #2504

[tonyandrewmeyer]

@dwilding and @james-garner-canonical: in the issue, James suggested using pathops, which handles pull correctly. I'm not sure how much we are using pathops within our docs - I imagine not in reference docs, but maybe in the how-to guides? If we would rather adapt the bits of doc to use that approach, I'm happy to do that.

[james-garner-canonical]

The changes to the docstrings and the custom notices example are a nice fix, thanks!

The changes to the managing files in the workload container guide clash with #2470. I'd suggest dropping the changes to the file made in this PR, and adding any that are relevant to #2470 (though I'm not sure if any of them are since we're switching to pathops for most examples -- but maybe worth mentioning the file closing issue doesn't affect pathops?).

Other suggestions are all non-blocking.

[tonyandrewmeyer]

The changes to the managing files in the workload container guide clash with #2470.

Ah yes, the PR I keep forgetting that I have only half-reviewed. Fixed now! Thanks for calling that out.