Docs: endian_reverse unclear

Created by: ax3l

Migrated from here so it does not get lost.

The CMake flag ADIOS2_USE_ENDIAN_REVERSE=ON looks scary to me for the following reason: if a file was written in one endinaness and read again in another, the BP file format should encode that and our BPEngine could automatically take care of it. (E.g. HDF5 does it that way).

I was working/interning a few years ago in Juelich where serveral big and little endian machines were connected to the same file system and workflows going back and forth were common. Using various adios modules, depending on the origin of the data, would be very complicated.

If it's not possible to already express this in BP itself, which I think it should be for portability, then we should at least make this a runtime option for now.