Add warning about using UPDATE query with functions to documentation

[elenatorro]

Related issue https://github.com/CartoDB/support/issues/2046

We need to add to the Data Services documentation a warning about what happens if you use an UPDATE query and it fails (basically, you may lose your data)

In addition, we should discuss if it'd be possible to use the Exception-Safe functions to avoid the problem above.

[jgoizueta]

This is tricky.

If credit-consuming functions are used in builder/dashboard custom sql windows in SELECTs they get executed multiple times.
So we could advise users to put them only in UPDATE/INSERTs. This is good advice in general because that's a way to keep what they paid for. And (I think) it solves the builder/dashboard problem because UPDATES/INSERTs are not executed multiple times (is that so?).

But, if you're going to use credit-consuming functions in UPDATE/INSERTs then your best served with the undocumented _exception_safe variants of the functions because otherwise, if they run out of credits amid the query execution they will get nothing (transaction is rolled-back) for their spent credits.

So, should we make the _exception_save functions the default (and document them) and un-document the others? Well, I'm not sure at all. For example if you use the functions to request a single result, e.g.

SELECT cdb_geocode_street_point('651 Lombard Street', 'San Francisco', 'California', 'United States')

I would prefer to see an error if I'm out of quota.