Clarify proper type casting for Numbers

[DannyNemer]

Clarifies when to use Number vs. parseInt for Number type casting, encouraging the proper usage of both.

Fixes: #523

Previously, at item 21.3, the guide read:

Use parseInt for Numbers and always with a radix for type casting.

However, the example displays:

// good
const val = Number(inputValue);

// good
const val = parseInt(inputValue, 10);

Issue: It is unclear whether Number is acceptable because the guide only directly states to use parseInt despite the example for Number. Indeed, Number and parseInt are not interchangeable and should both be used properply.

[ljharb]

could this say parsing strings, and also continue to say always with a radix?

[DannyNemer]

Done!

[ljharb]

👍 this LGTM