libbinder: emphasize kernel state objs

ProcessState/IPCThreadState objects are kernel objects,
and they are not used for RPC binder. Though, because
they have general names (they were created before we
co-opted userspace binder on top of sockets), this
deserves a comment explanation.

Bug: 237245600
Test: N/A
Change-Id: I914fa0578bac81fa131c8a1c1e28a99ca6b781bc