Outdated comment on scram_build_secret

Hi,

The documentation for the iterations parameter on scram_build_secret say:
* If iterations is 0, default number of iterations is used.

But the body of the function have the following assert
Assert(iterations > 0);

Commit b577743000 introduced the assert and remove the if condition.
All usages of this function is using an iteration > 0, so I think that
is just a matter of updating the documentation? If that's the case the
attached patch does that.

--
Matheus Alcantara
EDB: https://www.enterprisedb.com