Docs : Memory management for events : explain window:time, window:length

Issue Description:

The section "Memory management for events" is not clear about window:time and window:length.

https://docs.drools.org/latest/drools-docs/docs-website/drools/rule-engine/index.html#cep-memory-management-con_rule-engine

Acceptance Criteria:
Add explanations.
window:time -> calculate expiration
window:length -> doesn't calculate expiration