Documentation: highlight v3 post migration

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

Documentation/upgrades/upgrade_3_0.md

@@ -8,6 +8,8 @@ Before [starting an upgrade](#upgrade-procedure), read through the rest of this
 
 ### Upgrade checklists
 
+**NOTE:** When [migrating from v2 with no v3 data](https://github.com/coreos/etcd/issues/9480), etcd server v3.2+ panics when etcd restores from existing snapshots but no v3 `ETCD_DATA_DIR/member/snap/db` file. This happens when the server had migrated from v2 with no previous v3 data. This also prevents accidental v3 data loss (e.g. `db` file might have been moved). etcd requires that post v3 migration can only happen with v3 data. Do not upgrade to newer v3 versions until v3.0 server contains v3 data.
+
 #### Upgrade requirements
 
 To upgrade an existing etcd deployment to 3.0, the running cluster must be 2.3 or greater. If it's before 2.3, please upgrade to [2.3](https://github.com/coreos/etcd/releases/tag/v2.3.8) before upgrading to 3.0.

Documentation/upgrades/upgrade_3_1.md

@@ -8,6 +8,8 @@ Before [starting an upgrade](#upgrade-procedure), read through the rest of this
 
 ### Upgrade checklists
 
+**NOTE:** When [migrating from v2 with no v3 data](https://github.com/coreos/etcd/issues/9480), etcd server v3.2+ panics when etcd restores from existing snapshots but no v3 `ETCD_DATA_DIR/member/snap/db` file. This happens when the server had migrated from v2 with no previous v3 data. This also prevents accidental v3 data loss (e.g. `db` file might have been moved). etcd requires that post v3 migration can only happen with v3 data. Do not upgrade to newer v3 versions until v3.0 server contains v3 data.
+
 #### Monitoring
 
 Following metrics from v3.0.x have been deprecated in favor of [go-grpc-prometheus](https://github.com/grpc-ecosystem/go-grpc-prometheus):

Documentation/upgrades/upgrade_3_2.md

@@ -8,6 +8,8 @@ Before [starting an upgrade](#upgrade-procedure), read through the rest of this
 
 ### Upgrade checklists
 
+**NOTE:** When [migrating from v2 with no v3 data](https://github.com/coreos/etcd/issues/9480), etcd server v3.2+ panics when etcd restores from existing snapshots but no v3 `ETCD_DATA_DIR/member/snap/db` file. This happens when the server had migrated from v2 with no previous v3 data. This also prevents accidental v3 data loss (e.g. `db` file might have been moved). etcd requires that post v3 migration can only happen with v3 data. Do not upgrade to newer v3 versions until v3.0 server contains v3 data.
+
 Highlighted breaking changes in 3.2.
 
 #### Change in default `snapshot-count` value

Documentation/upgrades/upgrade_3_3.md

@@ -8,6 +8,8 @@ Before [starting an upgrade](#upgrade-procedure), read through the rest of this
 
 ### Upgrade checklists
 
+**NOTE:** When [migrating from v2 with no v3 data](https://github.com/coreos/etcd/issues/9480), etcd server v3.2+ panics when etcd restores from existing snapshots but no v3 `ETCD_DATA_DIR/member/snap/db` file. This happens when the server had migrated from v2 with no previous v3 data. This also prevents accidental v3 data loss (e.g. `db` file might have been moved). etcd requires that post v3 migration can only happen with v3 data. Do not upgrade to newer v3 versions until v3.0 server contains v3 data.
+
 Highlighted breaking changes in 3.3.
 
 #### Change in `etcdserver.EtcdServer` struct

Documentation/upgrades/upgrade_3_4.md

@@ -8,6 +8,8 @@ Before [starting an upgrade](#upgrade-procedure), read through the rest of this
 
 ### Upgrade checklists
 
+**NOTE:** When [migrating from v2 with no v3 data](https://github.com/coreos/etcd/issues/9480), etcd server v3.2+ panics when etcd restores from existing snapshots but no v3 `ETCD_DATA_DIR/member/snap/db` file. This happens when the server had migrated from v2 with no previous v3 data. This also prevents accidental v3 data loss (e.g. `db` file might have been moved). etcd requires that post v3 migration can only happen with v3 data. Do not upgrade to newer v3 versions until v3.0 server contains v3 data.
+
 Highlighted breaking changes in 3.4.
 
 #### Change in `etcd` flags