treewide: update docs file extensions (.txt -> .rst) in comments

treewide: update docs file extensions (.txt -> .rst) in comments

Several source comments still refer to docs with the old .txt
extension that were previously converted to reStructuredText.

Update these references to use the correct .rst extensions to
maintain accurate in-tree documentation pointers.

No functional changes.

Related commits:
  50f8174c5c1 (Jul 2021): docs/specs/acpi_nvdimm: Convert to rST
  f054eb1c920 (Jul 2021): docs/specs/acpi_pci_hotplug: Convert to rST
  912fb3678b8 (Sep 2023): docs/specs/vmgenid: Convert to rST
  bb1cff6ee04 (Sep 2023): docs/specs/ivshmem-spec: Convert to rST
  55ff468f781 (Jan 2022): docs: Rename ppc-spapr-hotplug.txt to .rst

Signed-off-by: Sean Wei <me@sean.taipei>
Message-ID: <20250616.qemu.relocated.05@sean.taipei>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>