Missing docs for SQL functions

If I'm not missing something completely obvious, the core tutorial at https://www.stackage.org/haddock/lts-10.10/esqueleto-2.5.3/Database-Esqueleto.html doesn't really teach how to use SQL functions/expressions, such as `date`, `random`, `now`, `substr`, etc.

Coupled with the fact that the modules `Database.Esqueleto.SQLite` and `Database.Esqueleto.MySQL` don't show up in the package contents, I was under the impression that Esqueleto had absolutely no way to groupBy `date(timestampCol)` in SQLIte. After going through https://github.com/bitemyapp/esqueleto/blob/master/src/Database/Esqueleto/PostgreSQL.hs I realise that `unsafeSQLFuntion` is that escape hatch.

- Are you open to a PR that documents this in the main tutorial?
- Are you open to PRs for adding more functions to https://github.com/bitemyapp/esqueleto/blob/master/src/Database/Esqueleto/SQLite.hs

Start with the core tutorial and compare the documented SQL expression guidance with Database/Esqueleto/PostgreSQL.hs, Database/Esqueleto/SQLite.hs, and Database/Esqueleto/MySQL.hs. Clarify how SQL functions such as date, random, now, and substr can be used, including the SQLite grouping example, and ensure the relevant modules and available functions are discoverable.

Written by the indexing model from the issue text.