Improve hint information related to duration phases

Specify that this field is for information purposes only and
does not automatically update phase status.

We also improve enable help text.

spec/system/admin/budget_phases_spec.rb

@@ -7,6 +7,10 @@ describe "Admin budget phases" do
     scenario "Update phase" do
       visit edit_admin_budget_budget_phase_path(budget, budget.current_phase)
 
+      expect(page).to have_content "These fields are used for information purposes only and do not trigger "\
+                                   "an automatic update of the active phase. In order to update it, edit "\
+                                   "the budget and select the active phase."
+
       fill_in "start_date", with: Date.current + 1.day
       fill_in "end_date", with: Date.current + 12.days
       fill_in_ckeditor "Description", with: "New description of the phase."