Added documentation for network registration state

Also renamed denial reason to reject cause to match
the 3GPP spec.

Bug: 73659459
Test: Build
Change-Id: Ia67ebf94771c7ff5f5d90f6cdd303cb2716f9186
(cherry picked from commit b4094993f742fbe9d07bd5783030aa82a3045f79)
2 files changed