Add `tag` to `listeners` documentation (#14803)

* Add `tag` to `listeners` documentation

* newsfile

changelog.d/14803.doc

@@ -0,0 +1 @@
+Add missing documentation for `tag` to `listeners` section.

docs/usage/administration/request_log.md

@@ -13,7 +13,7 @@ See the following for how to decode the dense data available from the default lo
 | Part  | Explanation |
 | ----- | ------------ |
 | AAAA  | Timestamp request was logged (not received) |
-| BBBB  | Logger name (`synapse.access.(http\|https).<tag>`, where 'tag' is defined in the `listeners` config section, normally the port) |
+| BBBB  | Logger name (`synapse.access.(http\|https).<tag>`, where 'tag' is defined in the [`listeners`](../configuration/config_documentation.md#listeners) config section, normally the port) |
 | CCCC  | Line number in code |
 | DDDD  | Log Level |
 | EEEE  | Request Identifier (This identifier is shared by related log lines)|

docs/usage/configuration/config_documentation.md

@@ -422,6 +422,10 @@ Sub-options for each listener include:
 
 * `port`: the TCP port to bind to.
 
+* `tag`: An alias for the port in the logger name. If set the tag is logged instead
+of the port. Default to `None`, is optional and only valid for listener with `type: http`.
+See the docs [request log format](../administration/request_log.md).
+
 * `bind_addresses`: a list of local addresses to listen on. The default is
        'all local interfaces'.