[Docs] Add note / warning of not using escape html settingl / modifier

[ampmonteiro]

Hi
in sequence of issue #863 .

I recommend that some pages / areas of the docs of v3 and v4, and even in future versions, should be updated with some kind of note / warning of not using or not be on the escape_html or $smarty->default_modifiers without a this option: ['escape:"htmlall"'] in order to help to prevent XSS attacks and use security good practice.

Example of some pages / areas in v4 docs:

• Getting started page
• in designers > language-basic-syntax > Introduction page
• in designers > language-variables > Introduction page
• in designers > language-modifiers > Introduction page
• in designers language-modifiers > Introduction page
• in designers language-modifiers > escape page
• in designers language-modifiers > unescape page
• in programmers > intro
• programmers/api-variables/variable-escape-html page