improve comments on find_package(Protobuf)

cmake/protobuf.cmake

@@ -53,6 +53,11 @@ if("${gRPC_PROTOBUF_PROVIDER}" STREQUAL "module")
   endif()
 elseif("${gRPC_PROTOBUF_PROVIDER}" STREQUAL "package")
   find_package(Protobuf REQUIRED ${gRPC_PROTOBUF_PACKAGE_TYPE})
+
+  # {Protobuf,PROTOBUF}_FOUND is defined based on find_package type ("MODULE" vs "CONFIG").
+  # For "MODULE", the case has also changed between cmake 3.5 and 3.6.
+  # We use the legacy uppercase version for *_LIBRARIES AND *_INCLUDE_DIRS variables
+  # as newer cmake versions provide them too for backward compatibility.
   if(Protobuf_FOUND OR PROTOBUF_FOUND)
     if(TARGET protobuf::${_gRPC_PROTOBUF_LIBRARY_NAME})
       set(_gRPC_PROTOBUF_LIBRARIES protobuf::${_gRPC_PROTOBUF_LIBRARY_NAME})