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GH-15084: [Ruby] Use common keys when keys.nil? in Table#join (#15088)

### What I did

* Add support for `keys = nil` in `Table#join`
  * When `keys.nil?` common keys in self and right will automatically set to keys.
* Add assertions without argument keys.
* Add missing comments for other overload cases.

### What I checked

* It works the same way in RedAmber.

### Related Issue

* Closes: #15084

Lead-authored-by: Hirokazu SUZUKI (heronshoes) <heronshoes877@gmail.com>
Co-authored-by: Sutou Kouhei <kou@clear-code.com>
Co-authored-by: Hirokazu SUZUKI <heronshoes877@gmail.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>