update: improve documentation for JS DCE (#1861)
update: improve documentation for JS DCE (#1861)

* Improve documentation for JS DCE
* Emphasize DCE application only for production assets
* Emphasize that names used as arguments should be in terms of the generated JS code
* Add note (+ warning) about how to keep whole packages / modules from elimination
JetBrains/kotlin-web-site,hltj/kotlin-web-site-cn,hltj/kotlin-web-site-cn,JetBrains/kotlin-web-site,JetBrains/kotlin-web-site,hltj/kotlin-web-site-cn,JetBrains/kotlin-web-site,hltj/kotlin-web-site-cn,hltj/kotlin-web-site-cn,JetBrains/kotlin-web-site,hltj/kotlin-web-site-cn,JetBrains/kotlin-web-site