FindMPI: add more search paths for MSMPI

Also comment what the various search paths are for.

Modules/FindMPI.cmake

@@ -174,7 +174,11 @@ set(_MPI_EXEC_NAMES                        mpiexec mpirun lamexec srun)
 # Grab the path to MPI from the registry if we're on windows.
 set(_MPI_PREFIX_PATH)
 if(WIN32)
+  # MSMPI
+  list(APPEND _MPI_PREFIX_PATH "$ENV{MSMPI_BIN}")
   list(APPEND _MPI_PREFIX_PATH "[HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\MPI;InstallRoot]/Bin")
+  list(APPEND _MPI_PREFIX_PATH "$ENV{MSMPI_INC}/..") # The SDK is installed separately from the runtime
+  # MPICH
   list(APPEND _MPI_PREFIX_PATH "[HKEY_LOCAL_MACHINE\\SOFTWARE\\MPICH\\SMPD;binary]/..")
   list(APPEND _MPI_PREFIX_PATH "[HKEY_LOCAL_MACHINE\\SOFTWARE\\MPICH2;Path]")
   list(APPEND _MPI_PREFIX_PATH "$ENV{ProgramW6432}/MPICH2/")