Version compatibility notes

While discussing embedding help for PostgreSQL sql language syntax into
pgAdmin III, we found it would be desirable if the latest documentation
includes some notes about backward compatibility for each command. This
could be in the "Compatibility" section, stating which minimum backend
version is required to support the command or option.

This shouldn't be too much work for the upcoming 7.4 documentation to
mark the 7.4 extensions. According to the results of the user survey, a
lot of users still have 7.2 running, so they might appreciate 7.3
extensions as well. This would cover >85 % of users.

Speaking specifically of pgAdmin III, only 7.3+ is supported, because we
rely on dependency tracking etc.

Regards,
Andreas