add explain about "user-scalable=no" and "maximum-scale=1.0" to section of <meta> element

[mtrootyy]

Validator.nu (X)HTML5 Validator(https://html5.validator.nu/) (https://checker.html5.org/) warn the following about "user-scalable=no" and "maximum-scale=1.0"

1.Warning: Consider avoiding viewport values that prevent users from resizing documents.
From line 5, column 1; to line 5, column 49
<meta name="viewport" content="user-scalable=no">
　
2.Warning: Consider avoiding viewport values that prevent users from resizing documents.
From line 6, column 1; to line 6, column 89
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0">

　
It is considered that explain about the above should be added to section of meta element in HTML Living Standard.