extras: protoc: document protoc trickiness
extras: protoc: document protoc trickiness

I tried to quickly go through the protoc source to find how the include
paths are interpreted, but I'm sleepy now.

hlange/LogSoCR,hlange/LogSoCR,hlange/LogSoCR,hlange/LogSoCR,hlange/LogSoCR,hlange/LogSoCR,hlange/LogSoCR,hlange/LogSoCR,hlange/LogSoCR,hlange/LogSoCR,hlange/LogSoCR,hlange/LogSoCR,hlange/LogSoCR