Update api_changes_list.md
Update api_changes_list.md

Mention that statistics classes aren't supposed to be used in regular plugins.
artspb/intellij-sdk-docs,artspb/intellij-sdk-docs,artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs,JetBrains/intellij-sdk-docs,JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs