x86/virt/tdx: Update the kexec section in the TDX documentation

TDX host kernel now supports kexec/kdump.  Update the documentation to
reflect that.

Opportunistically, remove the parentheses in "Kexec()" and move this
section under the "Erratum" section because the updated "Kexec" section
now refers to that erratum.

Signed-off-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Tested-by: Farrah Chen <farrah.chen@intel.com>
Link: https://lore.kernel.org/all/20250901160930.1785244-7-pbonzini%40redhat.com

Author: kaihuang

Documentation/arch/x86/tdx.rst

@@ -142,13 +142,6 @@ but depends on the BIOS to behave correctly.
 Note TDX works with CPU logical online/offline, thus the kernel still
 allows to offline logical CPU and online it again.
 
-Kexec()
-~~~~~~~
-
-TDX host support currently lacks the ability to handle kexec.  For
-simplicity only one of them can be enabled in the Kconfig.  This will be
-fixed in the future.
-
 Erratum
 ~~~~~~~
 
@@ -171,6 +164,13 @@ If the platform has such erratum, the kernel prints additional message in
 machine check handler to tell user the machine check may be caused by
 kernel bug on TDX private memory.
 
+Kexec
+~~~~~~~
+
+Currently kexec doesn't work on the TDX platforms with the aforementioned
+erratum.  It fails when loading the kexec kernel image.  Otherwise it
+works normally.
+
 Interaction vs S3 and deeper states
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~