docs: [VictoriaLogs] FAQ add logs without msg field (#7487)

### Describe Your Changes

Add FAQs to VictoriaLogs:
- I want to ingest logs without message field, is that possible?
- What if my logs have multiple message fields candidates

Preview:
https://github.com/VictoriaMetrics/VictoriaMetrics/blob/docs/VL-FAQ-empty-msg/docs/VictoriaLogs/FAQ.md#i-want-to-ingest-logs-without-message-field-is-that-possible

### Checklist

The following checks are **mandatory**:

- [x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).

docs/VictoriaLogs/FAQ.md

@@ -131,3 +131,42 @@ Just send the query with the needed [filters](https://docs.victoriametrics.com/v
 to [`/select/logsql/query`](https://docs.victoriametrics.com/victorialogs/querying/#querying-logs) - VictoriaLogs will return
 the requested logs as a [stream of JSON lines](https://jsonlines.org/). It is recommended specifying [time filter](https://docs.victoriametrics.com/victorialogs/logsql/#time-filter)
 for limiting the amounts of exported logs.
+
+## I want to ingest logs without message field, is that possible?
+
+Starting from version `v0.30.0`, VictoriaLogs started blocking the ingestion of logs **without a message field**, as it is a requirement of the [VictoriaLogs data model](https://docs.victoriametrics.com/victorialogs/keyconcepts/#message-field). 
+
+However, some logs do not have a message field and only contain other fields, such as logs in [this comment](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/7056#issuecomment-2434189718) and [this slack thread](https://victoriametrics.slack.com/archives/C05UNTPAEDN/p1730982146818249). Therefore, starting from version `v0.39.0`, logs without a message field are **allowed to be ingested**, 
+and their message field will be recorded as: 
+```json
+{"_msg": "missing _msg field; see https://docs.victoriametrics.com/victorialogs/keyconcepts/#message-field"}
+```
+
+The default message field value can be changed using the `-defaultMsgValue` flag, for example, `-defaultMsgValue=foo`.
+
+Please note that the message field is **crucial** for VictoriaLogs, so it is important to fill it with meaningful content.
+
+## What if my logs have multiple message fields candidates?
+
+When ingesting with VictoriaLogs, the message fields is specified through `_msg_field` param, which can accept **multiple fields**, and the **first non-empty field** will be used as the message field. 
+Here is an example URL when pushing logs to VictoriaLogs with Promtail:
+```yaml
+clients:
+  - url: http://localhost:9428/insert/loki/api/v1/push?_stream_fields=instance,job,host,app&_msg=message,body
+```
+
+For the following log, its `_msg` will be `foo bar in message`:
+```json
+{
+  "message": "foo bar in message",
+  "body": "foo bar in body"
+}
+```
+
+And for the following log, its `_msg` will be `foo bar in body`:
+```json
+{
+  "message": "",
+  "body": "foo bar in body"
+}
+```