Storage Account: Create backup policy overwrite change feed settings

I have a storage account whose blobservice has following properties set:

```json
{
"properties": {
"cors": {},
"deleteRetentionPolicy": {
"enabled": true,
"days": 7
},
"isVersioningEnabled": true,
"changeFeed": {
"enabled": true,
"retentionInDays": 7
},
"restorePolicy": {
"days": 6,
"enabled": true
},
"containerDeleteRetentionPolicy": {
"enabled": true,
"days": 7
}
}
}
```

Then I create a backup instance for this storage account with the policy defined as below:

```json
{
"properties": {
"datasourceTypes": [
"Microsoft.Storage/storageAccounts/blobServices"
],
"objectType": "BackupPolicy",
"policyRules": [
{
"isDefault": true,
"lifecycles": [
{
"deleteAfter": {
"duration": "P30D",
"objectType": "AbsoluteDeleteOption"
},
"sourceDataStore": {
"dataStoreType": "OperationalStore",
"objectType": "DataStoreInfoBase"
},
"targetDataStoreCopySettings": []
}
],
"name": "Default",
"objectType": "AzureRetentionRule"
}
]
}
}
```

After this, the blobsrevice is overwritten into:

```json
{
"sku": {
"name": "Standard_LRS",
"tier": "Standard"
},
"id": "xxxx",
"name": "default",
"type": "Microsoft.Storage/storageAccounts/blobServices",
"properties": {
"changeFeed": {
"enabled": true
},
"restorePolicy": {
"enabled": true,
"days": 30,
"minRestoreTime": "2023-03-06T03:15:07.9702151Z"
},
"containerDeleteRetentionPolicy": {
"enabled": true,
"days": 7
},
"cors": {
"corsRules": []
},
"deleteRetentionPolicy": {
"allowPermanentDelete": false,
"enabled": true,
"days": 35
},
"isVersioningEnabled": true
}
}
```

Changes include:

- `deleteRetentionPolicy`: 7 -> 35
- `changeFeed.retentionInDays`: 7 -> null
- `restorePolicy.days`: 6 -> 30 (what is set in the policy)

While `containerDeleteRetentionPolicy.days` is not changed (still 7)

## Expect Behavior

Clear documentation somewhere about above behavior

No repository file or test is named in the issue. Start by reviewing the Azure Storage blob service and backup policy behavior described in the examples, then identify the appropriate documentation location. Done means documenting why the retention and restore settings change, including the change-feed behavior and the unchanged container retention setting.

Written by the indexing model from the issue text.