Add note about new flavor ID validation

https://review.openstack.org/39441 introduce new
validation code for the flavor ID.

This patch adds a note to ensure people are using correct
numbers when manipulating flavor ids.

Change-Id: Iabecb974beb96da6e8610e097633ad9699655d3e
Closes-Bug: 1217238
Merge "Add note about new flavor ID validation"