Update description of parameter password_encryption

The previous description string still described the pre-PostgreSQL
10 (pre eb61136dc7) behavior of
selecting between encrypted and unencrypted, but it is now choosing
between encryption algorithms.

src/backend/utils/misc/guc.c

@@ -4729,10 +4729,8 @@ static struct config_enum ConfigureNamesEnum[] =
 
 	{
 		{"password_encryption", PGC_USERSET, CONN_AUTH_AUTH,
-			gettext_noop("Encrypt passwords."),
-			gettext_noop("When a password is specified in CREATE USER or "
-						 "ALTER USER without writing either ENCRYPTED or UNENCRYPTED, "
-						 "this parameter determines whether the password is to be encrypted.")
+			gettext_noop("Chooses the algorithm for encrypting passwords."),
+			NULL
 		},
 		&Password_encryption,
 		PASSWORD_TYPE_MD5, password_encryption_options,