docs: add description of single service IPI support

Add description of the HF_INTERRUPT_SEND_IPI paravirtualised
interface as well as the IPI feature itself. This patch covers
how an IPI is handled for a single service targeting a CPU.

Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>
Change-Id: I947ae9569e3cebcdc0fdf41d789607cc41eea285
diff --git a/docs/glossary.rst b/docs/glossary.rst
index d4e7280..1c28260 100644
--- a/docs/glossary.rst
+++ b/docs/glossary.rst
@@ -67,6 +67,9 @@
    GPT
       Granule Protection Table
 
+   IPI
+      Inter-Processor Interrupt
+
    ISA
       Instruction Set Architecture