Fix outdated comment of scram_build_secret()

This routine documented that "iterations" would use a default value if
set to 0 by the caller.  However, the iteration should always be set by
the caller to a value strictly more than 0, as documented by an
assertion.

Oversight in b577743000, that has made the iteration count of SCRAM
configurable.

Author: Matheus Alcantara
Discussion: https://postgr.es/m/ac858943-4743-44cd-b4ad-08a0c10cbbc8@gmail.com
Backpatch-through: 16

src/common/scram-common.c

@@ -200,8 +200,7 @@ scram_ServerKey(const uint8 *salted_password,
  *
  * The password should already have been processed with SASLprep, if necessary!
  *
- * If iterations is 0, default number of iterations is used.  The result is
- * palloc'd or malloc'd, so caller is responsible for freeing it.
+ * The result is palloc'd or malloc'd, so caller is responsible for freeing it.
  *
  * On error, returns NULL and sets *errstr to point to a message about the
  * error details.