linker: Add a comment regarding .bss placement.

kernel/linker.ld

@@ -40,6 +40,10 @@ SECTIONS
         *(.data .data.*)
     } :data
 
+    /* NOTE: .bss needs to be the last thing mapped to :data, otherwise lots of */
+    /* unnecessary zeros will be written to the binary. */
+    /* If you need, for example, .init_array and .fini_array, those should be placed */
+    /* above this. */
     .bss : {
         *(COMMON)
         *(.bss .bss.*)