add note to lines docs about empty str behavior

[msmoiz]

This pull request adds a short note to the documentation for str::lines that describes the behavior of the resulting iterator when called on an empty string. I tripped over this a few days ago because I thought (incorrectly) that the iterator would return a single line with an empty string. I don't doubt that the actual behavior (return no lines) is the correct behavior, but in the absence of explicit documentation describing it, I came to the wrong conclusion about it and maybe others will too. If this is so obvious as to be not worth including, I'm happy to withdraw the pull request! Thanks for taking a look.

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[Kivooeo]

imho, "no lines" feels not obvious what does that mean, like, would it be better to have wording like "returns None"?

[msmoiz]

Hmm, I think that might be a bit confusing since the method itself returns a Lines object instead of an Option and saying "returns None" makes it sound like it returns an Option. A few possible alternatives:

• "An empty string returns an empty iterator"
• "Returns an empty iterator when called on an empty string"
• "Empty strings produce no lines"

[Kivooeo]

Ah, sure it's an iterator

Empty iterator instead None then, should be clear enough

[msmoiz]

Okay cool, updated