Add two sections about type checks with generics and unchecked casts (#965)
Add two sections about type checks with generics and unchecked casts (#965)

* Add two sections about type checks with generics and unchecked casts.

* (minor) Reword a few statements

* Fix the problems found in the review

* (minor) Fix typo

hltj/kotlin-web-site-cn,JetBrains/kotlin-web-site,hltj/kotlin-web-site-cn,meilalina/kotlin-web-site,hltj/kotlin-web-site-cn,hltj/kotlin-web-site-cn,meilalina/kotlin-web-site,JetBrains/kotlin-web-site,meilalina/kotlin-web-site,meilalina/kotlin-web-site,JetBrains/kotlin-web-site,hltj/kotlin-web-site-cn,hltj/kotlin-web-site-cn,JetBrains/kotlin-web-site,JetBrains/kotlin-web-site,JetBrains/kotlin-web-site