Add Dataset#qualify_to and #qualify_to_first_source to sequel_3_dataset_methods extension

These methods are rarely use, and they can be completely
replaced by Dataset#qualify.
Minor updates to method lists in dataset basics guide
Deprecated using a mismatched number of placeholders and arguments in a placeholder literal string

A mismatched number of arguments is usually indicative of some
sort of logic error, and it's probably something Sequel should
complain about.

Issue pointed out by Jim Hranicky.