I'm OK with adding more cross-references (links or just brief comparative verbiage like the above) to non-SQL null value treatment but this document, for this patch, is going to focus solely on SQL NULL.
If you briefly mention JSON NULLs, it might be interesting to also briefly mention that in other places NULL can be seen differently, including changing its name. A SQL NULL passed as argument to a PL/Python function will be named None or PL/Perl will be undefined.
