ADIOS2: Document endian_reverse variant (#13226)
ADIOS2: Document endian_reverse variant (#13226)

Update the documentation of the `endian_reverse` variant in ADIOS2.
The short description caused some confusion (for me) :-)
iulian787/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack