Update coverage sample documentation.
Update coverage sample documentation.

We need to invoke `llvm-profdata` tool from
the same toolchain as Clang (on macOS we use the one from Xcode).

JetBrains/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native