Add #[doc(hidden)] to {vga_buffer, serial}::_print functions

src/serial.rs

@@ -10,6 +10,7 @@ lazy_static! {
     };
 }
 
+#[doc(hidden)]
 pub fn _print(args: ::core::fmt::Arguments) {
     use core::fmt::Write;
     use x86_64::instructions::interrupts;