sub-categorize special function forms

sub-categorize special function forms

this is the tutorial, which should have some semblence of
not getting too far into the weeds.  however, as we dont
really have other places to explain SQL concepts, and SQL
functions have a lot of them, we dont have another home right
now.  so at least further sub-categorize window functions,
table/column valued functions, and WITHIN GROUP into an
"advanced function techniques" section with a disclaimer that
these are less common use cases.

Change-Id: I4b16bd5673c0bd39a9b880338f6ce3cfbafbe271
(cherry picked from commit c7519e94b5f7bfd9cee908f80c793d628bc37a1c)