Bug 1262 - Document Transaction Chaining of MD-SAL

Improves and clarifies the transaction chain concept and related APIs.

Change-Id: Ib509cdef33f8b51f8bd142b93657eace489e03a1
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Bug 1262 - Document Transaction Chaining of MD-SAL"