[User story maps] Document that a single `|` produces an empty card

[ngirard]

I don't know whether it's an inadvertent behaviour or it's by design, but I discovered that when given a line containing a single |, TextUSM generates an empty card in user story maps.

I find the result more pleasant than when using e.g. a . as a fake title -- as I think you demonstrated somewhere in your examples. I think it's not documented anywhere, so I'm hereby suggesting to mention it in the docs.

Cheers