docs: $db->getFieldData() data explanation is not complete

[kenjis]

$db->getFieldData()'s explanation is not complete and accurate.

The following data is available from this function if supported by your database:

    name - column name
    max_length - maximum length of the column
    primary_key - 1 if the column is a primary key
    type - the type of the column

https://codeigniter4.github.io/CodeIgniter4/database/metadata.html#db-getfielddata