[DOCS] Documenting NULLVALUE() and NONNULLVALUE()

[David Garamond]

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?
--
dave
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