doc: fix flavor notes
doc: fix flavor notes

This fixes the two points in the note at the top of the
flavors page:

1. The policy rule in the first bullet was old so it's updated.

2. As of Ifa4e9cdfbbac1a1d4bf28679b24a17b13f637ddd in Pike,
   Horizon no longer allows you to 'edit' a flavor by default.
   Rather than try to explain historical bad behavior and the
   new default behavior in the Dashboard, the second bullet is
   simply removed.

Closes-Bug: #1715995

Change-Id: I372bf1e159d1db32461f843bd94c453d2e7df8d2
(cherry picked from commit a9f11002cc1b668b612a4ac7ce3d9662efbd2fd5)

vmturbo/nova,vmturbo/nova,vmturbo/nova,vmturbo/nova