Doc: sync src/tutorial/basics.source with SGML documentation.

Doc: sync src/tutorial/basics.source with SGML documentation.

basics.source is supposed to be pretty closely in step with
the examples in chapter 2 of the tutorial, but I forgot to
update it in commit f05a5e000.  Fix that, and adjust a couple
of other discrepancies that had crept in over time.

(I notice that advanced.source is nowhere near being in sync
with chapter 3, but I lack the ambition to do something
about that right now.)