Updated User & Plugin Guides: replaced "plugin docs <plugin_dist_name>" command with "openmdao docs
<plugin_dist_name>"; deleted confusing text in U.G. on viewing plugin docs.
Merge pull request #1270 from pziegfeld/fix_plugin_docs

Fixed confusing section in U.G. on viewing docs; replaced "plugin docs" with "openmdao docs" in U.G. & Plugin G.