Update "Combiner" docs for V3

[mtfishman]

Some updates to combiner docs (http://www.itensor.org/docs.cgi?vers=cppv3&page=classes/combiner) for V3:

• [x] rank -> order
• [x] C.m() -> dim(C)
• [x] Remove IndexName arg option.
• [x] Add example code of getting combined index with auto c = findIndex(C,"CMB")