docs: improve instance based documentation (#261)

* docs: improve clarity of usage notes for searxng & mastodon

* docs: further improve clarity

* refactor(spelling): Therfore -> Therefore

src/userstyles.yml

@@ -280,6 +280,9 @@ userstyles:
         @-moz-document domain("mastodon.social"), domain("mstdn.social"), domain("fosstodon.org"), domain("piaille.fr"), domain("tech.lgbt") {
         ```
         If you don't see your Mastodon server URL, feel free to add it to the list with the `domain("<URL>")` syntax.
+        > [!WARNING]
+        > On update your custom `@-moz-document domain("url")` will be removed so will have to re applied upon update. 
+        > Therefore we strongly encorage users to contribute to the project with their instances.
       faq:
         - question: "**The theme does not look the same as the preview?**"
           answer: "Your Mastodon instance may be using its own custom CSS, which is changing the look of the theme."
@@ -352,6 +355,15 @@ userstyles:
     name: SearXNG
     category: search_engine
     readme:
+      usage: |+
+        This userstyle is applied by default on a few common Searxng server URLs, listed at the top:
+        ```css
+        @-moz-document url-prefix("https://searx.baczek.me/"), url-prefix("https://searx.be/") {
+        ```
+        If you don't see your Searxng server URL, feel free to add it to the list with the `domain("<URL>")` syntax.
+        > [!WARNING]
+        > On update your custom `@-moz-document domain("url")` will be removed so will have to re applied upon update. 
+        > Therefore we strongly encorage users to contribute to the project with their instances.
       app-link: "https://github.com/searxng/searxng"
       current-maintainers: [*sekki21956, *ryanccn]
   startpage: