Document Strided Structured Material Sets

[JustinPrivitera]

We should document the information here:
https://github.com/LLNL/conduit/blob/dffc5a6c7b433665399231aa537469be32bce865/src/libs/relay/conduit_relay_io_silo.cpp#L2655-L2665

specifically the fact that strided structured mesh material sets only provide material data for zones that are actually in use, not for all zones.

[JustinPrivitera]

put this in the blueprint spec docs