Docs: multipath.conf.annotated: Update `path_selector' description

Docs: multipath.conf.annotated: Update `path_selector' description

In the following commit:

  c015b128103e7a6426d124a38cd679a181573b88
  multipath: change default path_selector to
  Date: Sat Jan 12 00:04:40 2013 -0600

the default for the `path_selector' attribute is changed
from "round-robin 0" to "service-time 0"; this new commit
reflects that change in `multipath.conf.annotated'.

Also, this commit adds documentation about the other
possible values for `path_selector'.

Signed-off-by: Michael Witten <mfwitten@gmail.com>