x86: Add a comment for board_init_f_r_trampoline()

Add a comment for this function in the header.

Change the function (and the one after) to use __noreturn to keep
checkpatch happy.

Add docs to board_init_f_r() while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/x86/include/asm/u-boot-x86.h b/arch/x86/include/asm/u-boot-x86.h
index 8f38c2d..1d8aa32 100644
--- a/arch/x86/include/asm/u-boot-x86.h
+++ b/arch/x86/include/asm/u-boot-x86.h
@@ -102,8 +102,20 @@
  */
 int fsp_save_s3_stack(void);
 
-void	board_init_f_r_trampoline(ulong) __attribute__ ((noreturn));
-void	board_init_f_r(void) __attribute__ ((noreturn));
+/**
+ * board_init_f_r_trampoline() - jump to relocated address with new stack
+ *
+ * @sp: New stack pointer to use
+ */
+void __noreturn board_init_f_r_trampoline(ulong sp);
+
+/**
+ * board_init_f_r() - jump to relocated U-Boot
+ *
+ * This is used to jump from pre-relocation to post-relocation U-Boot. It
+ * enables the cache and jump to the new location.
+ */
+void __noreturn board_init_f_r(void);
 
 int arch_misc_init(void);