Documentation: e1000: Update kernel documentation

Updated the e1000.txt kernel documentation with the latest information.

Also convert the text file to reStructuredText (RST) format, since the
Linux kernel documentation now uses this format for documentation.

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>