Explicitly mention which resources support compression in the config guide (matrix-org#13221)

Author: babolivier

changelog.d/13221.doc

@@ -0,0 +1 @@
+Document which HTTP resources support gzip compression.

docs/usage/configuration/config_documentation.md

@@ -486,7 +486,8 @@ Sub-options for each listener include:
 
    * `names`: a list of names of HTTP resources. See below for a list of valid resource names.
 
-   * `compress`: set to true to enable HTTP compression for this resource.
+   * `compress`: set to true to enable gzip compression on HTTP bodies for this resource. This is currently only supported with the
+     `client`, `consent` and `metrics` resources.
 
 * `additional_resources`: Only valid for an 'http' listener. A map of
    additional endpoints which should be loaded via dynamic modules.