add changes in RecurrenceGoal to 2.19 upgrade guide

docs/upgrade-guides/2-18-0-to-2-19-0.md

@@ -11,6 +11,19 @@ This is an early-release feature which may see some significant changes in futur
 Two new database migrations have been added to support Procedural Scheduling. If you are upgrading from a past version and want to preserve your data, follow the instructions in the
 [Database Migrations Guide](../deployment/advanced-database-migrations.mdx) to apply migrations after upgrading. Make sure to either pass the `--all` flag, or apply **both** migrations interactively.
 
+## Breaking Change to RecurrenceGoal functionality
+The behavior of the `RecurrenceGoal` has been changed when setting its `interval` parameter.
+
+When setting the `interval` parameter to 1 hour, the old behavior would create 1-hour bins for the duration of the horizon (or the more restrictive applyWhen period) and place 1 activity in each of these bins, whenever it is possible considering global scheduling conditions, behaviour which could have resulted in intervals of >1 hour between consecutive activities. With the new behavior, the `interval` parameter means a strict 1-hour distance between consecutive activities.
+
+New parameters have been introduced to give more control over the behavior of the goal and how it is solved:
+- A user can now give lower (`separatedByAtLeast`) and upper bounds (`separatedByAtMost`)  on the separation between activities. Only the upper bound will lead to conflicts being created. In other words, having activities separated by less than `separatedByAtLeast` will not create a conflict. But, when a conflict exists, `separatedByAtLeast` will be used for solving.
+- As said above, the old `interval` parameter has been left but setting it to X is equivalent to setting both `separatedByAtMost` and `separatedByAtLeast` to X.
+- The user can either use the `interval` parameter or the 2 new bounds parameters.
+- Another new parameter `previousActivityStartedAt` allows the user to specify when the “last” activity happened. It can potentially be a negative duration if it is before the start of the planning horizon. If not set, this parameter is set to `-separatedByAtMost`.
+
+The solver will now try several combinations to solve this goal, starting with the least number of activities possible (separated by a greater distance), and increase the number of activities until either all the temporal constraints are satisfied or it reaches a dead-end.
+
 ## Scheduling plan spec schema refactor (breaking change)
 
 The primary key for `scheduling_specification_goals` has been updated to a new unique auto-incrementing integer: `goal_invocation_id`. This is to allow multiple "invocations" of the same goal definition in a plan spec.