doc: Add documentation for bootloader_{type,version}

Add documentation for kernel/bootloader_type and
kernel/bootloader_version to sysctl/kernel.txt.  This should really
have been done a long time ago.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Shen Feng <shen@cn.fujitsu.com>

Documentation/sysctl/kernel.txt

@@ -19,6 +19,8 @@ Currently, these files might (depending on your configuration)
 show up in /proc/sys/kernel:
 - acpi_video_flags
 - acct
+- bootloader_type	     [ X86 only ]
+- bootloader_version	     [ X86 only ]
 - callhome		     [ S390 only ]
 - auto_msgmni
 - core_pattern
@@ -93,6 +95,35 @@ valid for 30 seconds.
 
 ==============================================================
 
+bootloader_type:
+
+x86 bootloader identification
+
+This gives the bootloader type number as indicated by the bootloader,
+shifted left by 4, and OR'd with the low four bits of the bootloader
+version.  The reason for this encoding is that this used to match the
+type_of_loader field in the kernel header; the encoding is kept for
+backwards compatibility.  That is, if the full bootloader type number
+is 0x15 and the full version number is 0x234, this file will contain
+the value 340 = 0x154.
+
+See the type_of_loader and ext_loader_type fields in
+Documentation/x86/boot.txt for additional information.
+
+==============================================================
+
+bootloader_version:
+
+x86 bootloader version
+
+The complete bootloader version number.  In the example above, this
+file will contain the value 564 = 0x234.
+
+See the type_of_loader and ext_loader_ver fields in
+Documentation/x86/boot.txt for additional information.
+
+==============================================================
+
 callhome:
 
 Controls the kernel's callhome behavior in case of a kernel panic.