api_changes_list_2018.md: update Kotlin nullability descriptions
api_changes_list_2018.md: update Kotlin nullability descriptions

JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs,JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs,artspb/intellij-sdk-docs,artspb/intellij-sdk-docs