nova: Add explanation for `Kalloc::phys2virt` and `virt2phys`

nova/kern/include/kalloc.h

@@ -38,7 +38,20 @@ class Kalloc
         void * alloc_page (unsigned size, Fill fill = NOFILL);
         void free_page (void *);
 
+        /**
+         * Even kernel can only access virtual memory, it cannot access physical memory directly.
+         * However, CPU and bootloader work with physical addresses, so we sometimes need to read/write
+         * to physical addresses.
+         *
+         * To resolve this, part of the physical memory is mapped 1:1 to a sequential block of virtual
+         * memory addresses at a known offset `OFFSET`, so that the physical address X is accessible
+         * at the virtual address OFFSET + X. This mapping is configured during kernel startup at
+         * `kern/src/start.S` (section `SETUP BOOT PAGE TABLE`).
+         *
+         * This method returns the virtual address that can be used to access the physical address `phys`.
+         */
         static void * phys2virt (mword);
 
+        /** Return the physical address that is mapped from the virtual address `virt` (opposite of `phys2virt`). */
         static mword virt2phys (void *);
 };