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>

Documentation/networking/index.rst

@@ -11,6 +11,7 @@ Contents:
    can
    dpaa2/index
    e100
+   e1000
    kapi
    z8530book
    msg_zerocopy