Documentation: PCI: Document PCIe TLP Header decoder for AER messages

The prefix/header of a TLP that caused an error may be recorded in the AER
Capability and emitted to the kernel log in raw hex format.  Document the
existence and usage of tlp-tool, which decodes the TLP Header into
human-readable form.

The TLP Header hints at the root cause of an error, yet is often ignored
because of its seeming opaqueness.  Instead, PCIe errors are frequently
worked around by a change in the kernel without fully understanding the
actual source of the problem.  With more documentation on available tools
we'll hopefully come up with better solutions.

There are also wireshark dissectors for TLPs, but it seems they expect a
complete TLP, not just the header, and they cannot grok the hex format
emitted by the kernel directly.  tlp-tool appears to be the most cut and
dried solution out there.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: Maciej Grochowski <mx2pg@pm.me>
Link: https://patch.msgid.link/bf826c41b4c1d255c7dcb16e266b52f774d944ed.1774246067.git.lukas@wunner.de