Elaborate on possible use of status codes with the errors middleware

docs/content/contributing/documentation.md

@@ -14,10 +14,10 @@ This [documentation](https://doc.traefik.io/traefik/) is built with [mkdocs](htt
 
 ### Method 1: `Docker` and `make`
 
-You can build the documentation and test it locally (with live reloading), using the `docs` target:
+You can build the documentation and test it locally (with live reloading), using the `docs-serve` target:
 
 ```bash
-$ make docs
+$ make docs-serve
 docker build -t traefik-docs -f docs.Dockerfile .
 # […]
 docker run  --rm -v /home/user/go/github/traefik/traefik:/mkdocs -p 8000:8000 traefik-docs mkdocs serve
@@ -85,10 +85,12 @@ Running ["HtmlCheck", "ImageCheck", "ScriptCheck", "LinkCheck"] on /app/site/bas
     If you've made changes to the documentation, it's safter to clean it before verifying it.
 
     ```bash
-    $ make docs-clean docs-verify
+    $ make docs
     ...
     ```
 
+    Will perform all necessary steps for you.
+
 !!! note "Disabling Documentation Verification"
 
     Verification can be disabled by setting the environment variable `DOCS_VERIFY_SKIP` to `true`:

docs/content/middlewares/errorpages.md

@@ -99,7 +99,10 @@ The status code ranges are inclusive (`500-599` will trigger with every code bet
 
 !!! note ""
 
-    You can define either a status code as a number (`500`) or ranges by separating two codes with a dash (`500-599`).
+    You can define either a status code as a number (`500`),
+    as multiple comma-separated numbers (`500,502`),
+    as ranges by separating two codes with a dash (`500-599`),
+    or a combination of the two (`404,418,500-599`).
 
 ### `service`