initrd.4, proc.5, bootparam.7: Update pointer to in-kernel initrd documentation

Linux commit v4.10-rc1~40^2~86^2~4 moves initrd documentation from
Documentation/initrd.txt to Documentation/admin-quide/initrd.rst.

* man4/initrd.4 (.SS Changing the normal root filesystem,
  .SH SEE ALSO): Amend pointer to in-kernel initrd documentation
  with change introduced in Linux 4.10 (move to
  Documentation/admin-guide/initrd.rst).
* man5/proc.5 (.SS Files and directories)
  <.TP .I /proc/sys/kernel/real-root-dev>: Likewise.
* man7/bootparam.7 (.SS Boot arguments for ramdisk use)
  <.TP .B 'noinitrd'>: Likewise.

Signed-off-by: Eugene Syromyatnikov <evgsyr@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man4/initrd.4

@@ -312,7 +312,11 @@ The use of
 .I /proc/sys/kernel/real-root-dev
 to change the root filesystem is obsolete.
 See the Linux kernel source file
+.I Documentation/admin\-guide/initrd.rst
+.\" commit 9d85025b0418163fae079c9ba8f8445212de8568
+(or
 .I Documentation/initrd.txt
+before Linux 4.10)
 as well as
 .BR pivot_root (2)
 and
@@ -486,6 +490,10 @@ The behavior may change in future versions of the Linux kernel.
 .BR freeramdisk (8),
 .BR rdev (8)
 .PP
+.I Documentation/admin\-guide/initrd.rst
+.\" commit 9d85025b0418163fae079c9ba8f8445212de8568
+(or
 .I Documentation/initrd.txt
+before Linux 4.10)
 in the Linux kernel source tree, the LILO documentation,
 the LOADLIN documentation, the SYSLINUX documentation

man5/proc.5

@@ -4627,7 +4627,11 @@ This value is the default if the kernel was not configured with
 .TP
 .I /proc/sys/kernel/real-root-dev
 This file is documented in the Linux kernel source file
-.IR Documentation/initrd.txt .
+.I Documentation/admin\-guide/initrd.rst
+.\" commit 9d85025b0418163fae079c9ba8f8445212de8568
+(or
+.I Documentation/initrd.txt
+before Linux 4.10).
 .TP
 .IR /proc/sys/kernel/reboot-cmd " (Sparc only) "
 This file seems to be a way to give an argument to the SPARC

man7/bootparam.7

@@ -378,7 +378,11 @@ the usual boot sequence (e.g., invocation of
 is performed.
 .IP
 For a detailed description of the initrd feature, see the kernel source file
-.IR Documentation/initrd.txt .
+.I Documentation/admin\-guide/initrd.rst
+.\" commit 9d85025b0418163fae079c9ba8f8445212de8568
+(or
+.I Documentation/initrd.txt
+before Linux 4.10).
 .IP
 The 'noinitrd' option tells the kernel that although it was compiled for
 operation with initrd, it should not go through the above steps, but