Clarify usage of date_part() for timespan data type.

Description of data changes visibility added.

from Constantin...

This patch fixes a few results in DatabaseMetaData, and updates the README
and TODO files (the later being a new file).

The TODO file lists the things that need to be looked into after 6.2 is
released, and describes the problem with Large Objects.

I asked for this a long time ago and Frank Dana recently has
indicated that using lbsd on aix breaks the timezone code.

I've found a problem in the Postgresql jdbc driver.
"ReceiveInteger" shifts a received byte right instead of left.
This means that only the least significant byte is read into the int.

Reviewed by: Peter T Mount <patches@maidast.demon.co.uk>

Rearrange the v6.2 enhancements section to put visible changes first.

Add documentation of some text functions.
Add descriptions of some new SQL/92-compatible string functions.

Create explicit drop_table man page (copied from old drop man page).