Update documentation for issue 21986

Small doc update, so that users of the documentation can easier understand what are database indexes.
I'm not too familiar with django, so there could be a better link that wiki.


To test out my change, perform following steps:
# Checkout my change
# go to docs folder && run make html
# Open file _build/html/topics/db/optimization.html in a browser

Cavet: This is my first pull request, let me know if I've missed something.