New manual chapters

Hey Peter,

I notice that here:

http://candle.pha.pa.us/main/writings/pgsql/sgml/ddl-alter.html

You mention that you cannot remove a column. This is no longer true as of
last friday.

And here:

http://candle.pha.pa.us/main/writings/pgsql/sgml/ddl-depend.html

You say:

"All drop commands in PostgreSQL support specifying CASCADE. Of course"

Which isn't technically true for the ALTER TABLE/DROP NOT NULL statement..

Chris