samples: net: Fix README.rst file documentation

Various network samples contained QEMU slip setup instructions
or those instructions were missing. A reference doc in
doc/subsystems/networking/qemu_setup.rst file already has the
setup instructions for QEMU. So add a reference to that file
in samples/net/*/README.rst files and remove unnecessary slip
setup instructions in relevant files.

Fix various typos in readme files at the same time.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>