[defns.valid] Elaborate on cause in example and add cross-references

[Eisenwave]

To be honest, all I wanted to do is add a forward reference to [lib.types.movedfrom] to make the relevant section in the library intro easier to find, but that also required some wording changes to the example to make it less awkward.

[jensmaurer]

I think it's irrelevant how the "valid but unspecified state" was reached; you can always call x.empty(). Thus, I think you can just add the list of cross-references and avoid the rephrasing.

[Eisenwave]

I think it's irrelevant how the "valid but unspecified state" was reached; you can always call x.empty(). Thus, I think you can just add the list of cross-references and avoid the rephrasing.

That is true, but keep in mind that this is updating an example. It's not necessary to keep it general; the reader can just infer that this is one possible way that the state is reached.

Also, this allows us to add more cross-references to sections related to move operations, which I see as a slight plus.

If you insist, I can cut the PR down to just adding \iref{lib.types.movedfrom}.

[jensmaurer]

Your rephrasing gives me the impression that it's important that the "valid but unspecified state" was reached via a move operation, in order to be allowed to call x.empty(). I don't think that impression is helpful.

[Eisenwave]

@jensmaurer is it better with the insertion of "say"? This should clarify that move operations are just one way that the state could have been entered.

[jensmaurer]

I don't like "say" from a stylistic viewpoint.

You can add a second sentence, though (and leave the first one essentially alone): "The class std::vector is in a valid but unspecified state after being moved from (xrefs)."

[jwakely]

I think the original example was better by virtue of being simpler, and I don't think the cross-reference adds anything.