doc: add details for the unreviewed MR acceptance process

doc/acceptance.md

@@ -57,7 +57,7 @@ graph TD
     Q2 -->|Yes| Reviewable([Reviewable])
     Q2 -->|No| NotCompliant
     Q2 -->|Cannot determine| PipelineNotFinished([PipelineNotFinished])
-    Reviewable -->|"No vote and No thread opened during 72h"| Accepted([Accepted])
+    Reviewable -->|"No vote and No thread opened during 72h (1)"| Accepted([Accepted])
     Reviewable -->|"Thread opened or another Developer voted"| InReview([InReview])
     InReview -->|"All threads resolved and Score >= 0"| Acceptable([Acceptable])
     InReview -->|"Inactive for 60 days"| Stale([Stale])
@@ -73,6 +73,10 @@ style Reviewable color:#FF610A,stroke:#FF610A
 style Stale color:#6699cc,stroke:#6699cc
 ```
 
+*(1)*: 72h after the creation time of the MR, if there is no new version of it. The exact deadline is the later of:
+- `(time of the first version of the MR or out-of-draft time) + 72h`
+- `(time of the last "non simple/fast-forward rebase" version of the MR) + 24h`
+
 ### If the MergeRequest author is **NOT** a project's developer
 
 ```mermaid