Documenting NULLVALUE() and NONNULLVALUE()

Perl and PHP programmers (such as myself) are accustomed to using
defined()/isset() and so when wanting to test null value they might look
to the Functions chapter (9) instead of the SQL Syntax chapter (4). It
took me some time before I found out about ISNULL, IS NULL, NOTNULL, IS
NOT NULL. In MySQL I am accustomed to using ISNULL() function.

Perhaps we can change section 9.1 from "Logical Operators" to "Logical
Functions and Operators"? NULLVALUE() and NONNULLVALUE() can be
documented there, along with IS, IS NULL, and IS NOT NULL (which also
don't seem to have been covered yet in Chapter 9).

Should I submit a patch?

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dave