Docs: Add note about accumulate function return types

Issue Description:

Add note about accumulate function return types.

Basically `Comparable`. In addition, depending on functions, some specialized return types are available (e.g. `Integer`).

https://github.com/kiegroup/drools/blob/main/drools-compiler/src/main/java/org/drools/compiler/rule/builder/util/AccumulateUtil.java#L17-L53

Acceptance Criteria:

Add a note to clarify the above