Update docs to clarify the relationship between name and id

[YgorSouza]

The name passed to new() defines the initial id, but changing it with name() does not change the id, it has to be changed explicitly with id(). This documentation update attempts to make this caveat more explicit, as it is not apparent from the public API without looking at the source.

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[YgorSouza]

This tripped me when I was upgrading to v0.32. I thought I could just set the new name argument to an empty string everywhere since I was already using the name() method, and only later realized the legend was partially broken because all the items had the same ID. So I thought I'd add some documentation to explain it more clearly.

Should I add a docstring to every new() method explaining that as well? I'm not sure if that's overkill.