Fix comment in fe-auth-scram.c

Fix comment in fe-auth-scram.c

The frontend-side routine in charge of building a SCRAM verifier
mentioned that the restrictions applying to SASLprep on the password
with the encoding are described at the top of fe-auth-scram.c, but this
information is in auth-scram.c.

This is wrong since 8f8b9be, so backpatch all the way down as this is an
important documentation bit.

Spotted while reviewing a different patch.

Backpatch-through: 11