proc.5: Fixes various references to kernel docs in Documentation/

Especially the change to .rst format in the kernel Documentation/
tree has rendered many of the references in this manual page
obsolete. Fix them.

Reported-by: Vito Caputo <vcaputo@pengaru.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man5/proc.5

@@ -1519,8 +1519,11 @@ See
 for a description of these fields.
 Parsers should ignore all unrecognized optional fields.
 .IP
-For more information on mount propagation see:
+For more information on mount propagation see
+.I Documentation/filesystems/sharedsubtree.rst
+(or
 .I Documentation/filesystems/sharedsubtree.txt
+before Linux 5.8)
 in the Linux kernel source tree.
 .TP
 .IR /proc/[pid]/mounts " (since Linux 2.4.19)"
@@ -3159,7 +3162,10 @@ This can be used by MAKEDEV scripts for consistency with the kernel.
 .IR /proc/diskstats " (since Linux 2.5.69)"
 This file contains disk I/O statistics for each disk device.
 See the Linux kernel source file
+.I ./Documentation/admin\-guide/iostats.rst
+(or
 .I Documentation/iostats.txt
+before Linux 5.3)
 for further information.
 .TP
 .I /proc/dma
@@ -3831,9 +3837,11 @@ The format of this file is documented in
 .I /proc/mtrr
 Memory Type Range Registers.
 See the Linux kernel source file
+.I Documentation/x86/mtrr.rst
+(or
 .I Documentation/x86/mtrr.txt
 .\" commit 7225e75144b9718cbbe1820d9c011c809d5773fd
-(or
+before Linux 5.2, or
 .I Documentation/mtrr.txt
 before Linux 2.6.28)
 for details.
@@ -4375,7 +4383,10 @@ Using other separators leads to the error
 This directory may contain files with application binary information.
 .\" On some systems, it is not present.
 See the Linux kernel source file
+.I Documentation/sysctl/abi.rst
+(or
 .I Documentation/sysctl/abi.txt
+before Linux 5.3)
 for more information.
 .TP
 .I /proc/sys/debug
@@ -6843,9 +6854,9 @@ of thing that needs to be updated very often.
 .BR sysctl (8)
 .PP
 The Linux kernel source files:
-.IR Documentation/filesystems/proc.txt ,
-.IR Documentation/sysctl/fs.txt ,
-.IR Documentation/sysctl/kernel.txt ,
-.IR Documentation/sysctl/net.txt ,
+.IR Documentation/filesystems/proc.rst ,
+.IR Documentation/admin\-guide/sysctl/fs.rst ,
+.IR Documentation/admin\-guide/sysctl/kernel.rst ,
+.IR Documentation/admin\-guide/sysctl/net.rst ,
 and
-.IR Documentation/sysctl/vm.txt .
+.IR Documentation/admin\-guide/sysctl/vm.rst .