Improve the doc to explain the usage of variables *_ENABLED and *_DISABLED

Description

Many users misinterprets the variables of type *_ENABLED and *_DISABLED. We check the presence of variable, not the content. It's better to improve the usage doc and avoid the questions around it.

EX: NODE_LINT_ENABLED: 'false' --> Lint enabled due to variable, user might think lint disabled with value 'false'

Implementation ideas

Improve the ReadMe for all the templates which have variables *_ENABLED and **_DISABLED