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

打开 https://javascript.info/logical-operators 页面，并将其中对“falsy”的用法与 issue 中链接的 MDN Falsy glossary 进行比较。添加一段简短说明，解释 JavaScript 为什么使用“falsy”而不是“false”；该页面应为首次遇到这个术语的读者清楚地定义它。

由索引模型根据 Issue 内容生成。