Docs: add missing configuration option (#9092)

**What this PR does / why we need it**:
Promtail can set the tenant id from a label with PR 6290 (commit
a1e0298a5).
However, while docs/sources/clients/promtail/stages/tenant.md was
appropriately updated, docs/sources/clients/promtail/configuration.md
was left untouched.

**Which issue(s) this PR fixes**:

**Special notes for your reviewer**:

**Checklist**
- [ ] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`

docs/sources/clients/promtail/configuration.md

@@ -781,9 +781,13 @@ picking it from a field in the extracted data map.
 
 ```yaml
 tenant:
-  # Name from extracted data to whose value should be set as tenant ID.
-  # Either source or value config option is required, but not both (they
+  # Either label, source or value config option is required, but not all (they
   # are mutually exclusive).
+
+  # Name from labels to whose value should be set as tenant ID.
+  [ label: <string> ]
+
+  # Name from extracted data to whose value should be set as tenant ID.
   [ source: <string> ]
 
   # Value to use to set the tenant ID when this stage is executed. Useful