fix(docs): put INIT_UNUSED_NS_EL2 docs back

Commit b65dfe40a removed the documentation for this flag in error. Put
it back.

Change-Id: I61a352553a010385997c47116b53d2fbe939ccd4
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
1 file changed