daemon/session2.h: clarify `struct session2` docs

daemon/session2.h: clarify `struct session2` docs

As pointed out by @lukas.ondracek, the wording could be confusing when
mentioning *bottommost* and *topmost* layers. The original wording was
meant to reference the way the arrays in `daemon/session2.c` are laid
out, but never explicitly mentioned that, so one could be implicitly
think it was in reference to the widely known ISO/OSI or TCP/IP models,
which could be interpreted incorrectly (the layers are traditionally
laid out the other way around there).