Conventions are that column identifier should generally be double quoted. Not mandatory, but good practice to be consistent in the docs.

`array('type' => 'index|unique|fulltext', 'value' => '"FieldA","FieldB"')`

Same convention as other SQL statements (e.g. `WHERE "Column" = 'value'`)

I wouldn't mind to make reference to the string notation for column specification, since it may help developers working with the code, and help to avoid the issue you had to fix below. For now it's still a valid syntax, so it should be documented.

I agree that the array syntax should be the preferred approach, since it's more explicit.