Introduce SQL language "functions" first

[polobo]

I'd probably introduce SQL language "functions" first and describe them (and SQL generally) as being an imperative language.

Then when introducing pl/pgsql I'd highlight that it is a procedural language and should be chosen when SQL cannot solve the problem at hand - in particular since executing queries with SQL functions can be optimized moreso that queries with pl/pgsql functions.

I seem to recall "stored procedures" having a "standalone transaction context" associated with them while functions always execute within the same transaction as the caller. I'd change the name for 20_stored_procedures.sql for this reason; though I guess it depends on what the target audience is more likely to expect.

[kmoppel]

Thanks for the suggestion! Currently I just added your comments to "intro"...for splitting the two apart would probably need some more samples for pure SQL functions - do you have something in mind? A PR would be very welcome :)