Documentation: Add ITLB_MULTIHIT documentation

Add the initial ITLB_MULTIHIT documentation.

[ tglx: Add it to the index so it gets actually built. ]

Signed-off-by: Antonio Gomez Iglesias <antonio.gomez.iglesias@intel.com>
Signed-off-by: Nelson D'Souza <nelson.dsouza@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>