memory::paging: Add note that we use recursive P4 paging

src/arch/x86_64/memory/paging/mod.rs

@@ -1,4 +1,4 @@
-//! Paging subsystem.
+//! Paging subsystem. *Note: uses recursive mapping.*
 //!
 //! Extremely ripped off from Phil Oppermann's tutorials, because I don't feel like writing
 //! a paging system off the top of my head today.