Documentation could be more newbie-friendly

I found another thing that is a bit unclear in the documentation.
When using the "is_unique" validation rule in a Model, you apparently have to supply the table and field name that you want to be unique.

I assumed it to be obvious that the field you want to be checked for uniqueness is the field that you define the rule for ?
So the rule resulted in an error (about a null value being supplied to the is_unique() method as the 2nd argument), when I didn't supply <table>.<field>.

There may be good reasons to require explicitly stating <table>.<field>, but it would help if the documentation said so.

Where should it be documented?

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