Update documentation for issue 21986

Small doc update, so that users of the documentation can easier understand what are database indexes.
Issue link https://code.djangoproject.com/ticket/21986
I'm not too familiar with django, so there could be a better link that wiki.


To test out my change, perform following steps:
1) Checkout my change
2) go to docs folder && run make html
3) 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.