docs: fix checksrc warning, fix checkdocs CI filter

Also:
- GHA/checkdocs: fix CI filters to catch it early.
  Follow-up to 28dd14aafe #15797

Closes #19533

docs/libcurl/curl_multi_perform.md

@@ -85,8 +85,7 @@ int main(void)
         break;
       }
 
-    /* if there are still transfers, loop */
-    } while(still_running);
+    } while(still_running);  /* if there are still transfers, loop */
   }
 }
 ~~~