Link to DEFINE INDEX on DEFINE FIELD docs

[caleblamro]

Hi all! Today while defining a table, I wanted to make one of the fields ('name') unique. I intuitively went to the DEFINE FIELD docs, as I was aware of the ASSERT statement. I was able to make the field unique with ASSERT, but while searching some more, I found the DEFINE INDEX docs. I think there should be a section that describes making a field unique, but on the DEFINE FIELD docs so users who end up here get their question answered and are sent to the correct location. I'm more than willing to do this, but wanted to ask everyone's opinion before making a PR. Thank you!