5.1.1 -- last minute fixes

A few last-minute fixes to the 5.1.1 docs:

planning-guide/7000-nsx-plan -- state that NSX is experimental

release-notes/020-new-features -- made "(experimental)" consistent
in headers

release-notes/050-known-issues -- corrected some syntax in NSX section

terminology/experimental-features -- added xref to planning guide
for NSX

Change-Id: I350e4bc3b5a8c72442e5b6848a8cebaf0f32167f