Remove info on how to enable feature flags After Loading Ember.

Not all feature flags work when enabling them after loading Ember e.g.
string-humanize.

This is really misleading, making people believe they are doing
something wrong. In order to avoid that we should encourage people to
use the `ENV` object and enable them always before loading Ember.
Merge pull request #1039 from abuiles/edit_feature_flags_details

Remove info on how to enable feature flags After Loading Ember.