Update alarm expression BNF documentation

Change-Id: I3ebb993b4e1ef4942f29b00ed1f2c75c9b6d6ba5
Update alarm expression BNF documentation

Change-Id: Ic33cdadbd9d7d9402c860a8d17dd81c62decc744
Merge commit '4b70c20b45a4edc3b8e981fc9eb77b7ff4bf6861'