Introduce and explain the "falsy" term

Hello

Huge thanks for your valuable tutorial, I am re-working with JS and because JS itself evolves I am refreshing my knowledge with your tutorial

Just in case in the following page

* https://javascript.info/logical-operators

Appears many times the "falsy" term. I thought it was an error but actually it is a valid term in JS

It indicated at (and in other pages):

* https://developer.mozilla.org/en-US/docs/Glossary/Falsy

Therefore consider in add a special note indicating why is used the "falsy" term and not "false" (as is expected at first glance)

Thanks for your understanding

開啟 https://javascript.info/logical-operators 頁面，並將其中對「falsy」的用法與 issue 中連結的 MDN Falsy glossary 進行比較。新增一段簡短說明，解釋 JavaScript 為什麼使用「falsy」而不是「false」；該頁面應為首次遇到這個術語的讀者清楚地定義它。

由索引模型根據 Issue 內容生成。