docs(context-mgmt): add Root-Context documentation

* This patch adds some details on the EL3/Root-Context
  and its related interfaces.

* Additionally it updates the existing details on the
  interfaces, related to various CPU context entries which
  have been improvised recently.

Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
Change-Id: I81a992fe09feca4dc3d579a48e54a4763425e052
2 files changed