Glossary: update entry on the epoch structure

Author: amesgen

docs/website/contents/for-developers/Glossary.md

@@ -137,22 +137,27 @@ In the latest Cardano era, each epoch lasts for 432000 slots (= 5 days).
 
 The ledger rules take snapshots of the nonce and stake distribution at different points of time in the course of an epoch. A snapshot may only be _used_ when it has stabilized, which means that their block has become immutable (being older than `k` blocks). Currently, Cardano `mainnet` uses an epoch length of `10k/f` slots, divided into three parts:
 
-- Part 1, length `4k/f` - At the beginning of this part, which forms the boundary with the previous epoch, the stake distribution snapshot is taken.
+Note that nothing in the implementation happens on the transition from Part 1 to Part 2 (in contrast to "from Part 2 to Part 3"), so there exist no concrete values for the individual length of these two phases. The length of Part 3 is however explicitly recorded in the implementation, so the length of Part 1 and Part 2 combined is `10k/f` minus the length of Part 3.
+
+- Part 1, at least length `3k/f` - At the beginning of this part, which forms the boundary with the previous epoch, the stake distribution snapshot is taken.
   At the end of this part, the stake distribution has stabilized.
 
-- Part 2, length `3k/f` - At the end of this part, the nonce snapshot is taken.
+- Part 2, at least length `k/f` - At the end of this part, the nonce snapshot is taken.
 
   The nonce is snapshotted after the stake distribution to prevent _identity grinding_.
   However, this does not prevent _nonce grinding_.
 
   It must contain at least one honest block (created by an honest party as defined above), so that the nonce cannot be solely influenced by the adversary – otherwise they could identity grind before Part 1, knowing what the nonce would be due to owning all the blocks up to its snapshot.
   This criterion is a minimum requirement and wildly unrealistic not to be satisfied with `mainnet` parameters.
 
-  The nonce snapshot could likely be taken earlier without sacrificing security, like after Part 1.
-  Waiting another `3k/f` is playing it extra safe.
+  The nonce snapshot could likely be taken earlier without sacrificing security, like already after `3k/f` after the start of the epoch.
+  Waiting all the way until the start of Part 3 is playing it extra safe.
+
+- Part 3, length `4k/f` since the Conway era, and `3k/f` for all prior Shelley-based eras - At the end of this part, the nonce snapshot has stabilized and can be used for the leader schedule of the next epoch.
 
-- Part 3, length `3k/f` - At the end of this part, the nonce snapshot has stabilized and can be used for the leader schedule of the next epoch.
+  Most importantly, Part 3 has to be at least `3k/f` slots (one stability window) long for the nonce to stabilize before the start of the next epoch (such that all pools agree on the leader schedule).
 
+  As advised by the IOG researchers, the nonce should be snapshotted even a bit earlier for intricate reasons related to Ouroboros Genesis. See erratum 17.3 in the [Shelley ledger specs](https://github.com/IntersectMBO/cardano-ledger/blob/master/README.md) for context.
 
 ## :Eventual consensus