PCI: Remove no longer correct documentation regarding MSI vector assignment

The MSI vector reservation system described in Documentation/MSI-HOWTO.txt
was removed by Eric in 92db6d10bc1bc43330a4c540fa5b64c83d9d865f. Remove
the references to it in the documentation.

While we're here § 5.5.1 refers to x86 hardware requirements, so make that
clear.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>