Documentation for vfs/gc.c

[mooffie]

While there's some vague explanation about "stamping" in lib/vfs/HACKING, there isn't a concrete one. This patch fixes this.

(Why did I write it? I wanted LuaFS (the component in mc^{2 that lets people write filesystems in Lua) do be well documented, and there was no way to do this without also documenting these important C functions.)}