More notes for the upgrade guide (#5176)
More notes for the upgrade guide (#5176)

SAM improvements to Kotlin
Companion imports
R8 and ProGuard
square/okhttp,square/okhttp,ansman/okhttp,ze-pequeno/okhttp,yschimke/okhttp,cketti/okhttp,ansman/okhttp,ze-pequeno/okhttp,cmzy/okhttp,cmzy/okhttp,tiarebalbi/okhttp,tiarebalbi/okhttp,cketti/okhttp,tiarebalbi/okhttp,ansman/okhttp,yschimke/okhttp,ze-pequeno/okhttp,square/okhttp,cketti/okhttp,yschimke/okhttp