Much of what is in the Reference is auto-generated from comments in the source code, so you can’t make a PR. But why do you think the text should be changed. After all, you can define the content for multiple fields and multiple rows in a structure field, so that is not really the same.
but the part when a new page/file/user is created is the one I stumbled upon and you told me, this is the behaviour.
If it is exactly the same as for textfields, then it should be documented the same - at least that is what I am proposing. As for what value it is: the structure expects and array, where as textfield expects a string. which will be left unchanged.
Anyways, I now know how to use it - it was just a proposal on my side
Ah, now I see what you were referring to, a misunderstanding. Looks like it’s almost the only field where that bit is missing. In any case, that behavior is true for all defaults of all fields.