docs: fix broken documentation links

Mostly due to x86 and acpi conversion, several documentation
links are still pointing to the old file. Fix them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Reviewed-by: Wolfram Sang <wsa@the-dreams.de>
Reviewed-by: Sven Van Asbroeck <TheSven73@gmail.com>
Reviewed-by: Bhupesh Sharma <bhsharma@redhat.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/x86/x86_64/5level-paging.rst

@@ -20,7 +20,7 @@ physical address space. This "ought to be enough for anybody" ©.
 QEMU 2.9 and later support 5-level paging.
 
 Virtual memory layout for 5-level paging is described in
-Documentation/x86/x86_64/mm.txt
+Documentation/x86/x86_64/mm.rst
 
 
 Enabling 5-level paging

Documentation/x86/x86_64/boot-options.rst

@@ -9,7 +9,7 @@ only the AMD64 specific ones are listed here.
 
 Machine check
 =============
-Please see Documentation/x86/x86_64/machinecheck for sysfs runtime tunables.
+Please see Documentation/x86/x86_64/machinecheck.rst for sysfs runtime tunables.
 
    mce=off
 		Disable machine check
@@ -89,7 +89,7 @@ APICs
      Don't use the local APIC (alias for i386 compatibility)
 
    pirq=...
-	See Documentation/x86/i386/IO-APIC.txt
+	See Documentation/x86/i386/IO-APIC.rst
 
    noapictimer
 	Don't set up the APIC timer

Documentation/x86/x86_64/fake-numa-for-cpusets.rst

@@ -18,7 +18,7 @@ For more information on the features of cpusets, see
 Documentation/cgroup-v1/cpusets.txt.
 There are a number of different configurations you can use for your needs.  For
 more information on the numa=fake command line option and its various ways of
-configuring fake nodes, see Documentation/x86/x86_64/boot-options.txt.
+configuring fake nodes, see Documentation/x86/x86_64/boot-options.rst.
 
 For the purposes of this introduction, we'll assume a very primitive NUMA
 emulation setup of "numa=fake=4*512,".  This will split our system memory into