Document NULL

Hi,

Over in [1] it was rediscovered that our documentation assumes the reader
is familiar with NULL. It seems worthwhile to provide both an introduction
to the topic and an overview of how this special value gets handled
throughout the system.

Attached is a very rough draft attempting this, based on my own thoughts
and those expressed by Tom in [1], which largely align with mine.

I'll flesh this out some more once I get support for the goal, content, and
placement. On that point, NULL is a fundamental part of the SQL language
and so having it be a section in a Chapter titled "SQL Language" seems to
fit well, even if that falls into our tutorial. Framing this up as
tutorial content won't be that hard, though I've skipped on examples and
such pending feedback. It really doesn't fit as a top-level chapter under
part II nor really under any of the other chapters there. The main issue
with the tutorial is the forward references to concepts not yet discussed
but problem points there can be addressed.

I do plan to remove the entity reference and place the content into
query.sgml directly in the final version. It is just much easier to write
an entire new section in its own file.

David J.

[1] https://www.postgresql.org/message-id/1859814.1714532025%40sss.pgh.pa.us