Improve wording

lib/metadata/policies.js

@@ -792,7 +792,7 @@ var policies = {
     "compression": {
         "tool": "redownload",
         "label": "Brotli compression",
-        "message": "<p>Measures the number of bytes that could be saved by compressing textual files.</p><p>Gzip is a good old algorithm that offers great improvements. But Brotli is a new-generation algorithm and provides even better results. All major server systems are now compatible with Brotli.</p><p>Note that compressing small files (< 1 KB) is arguable, and that some assets such as images should not be gzipped as they are already compressed. <a href=\"https://gist.github.com/gmetais/971ce13a1fbeebd88445\" target=\"_blank\">Here</a> is a list of Content-Types that should be compressed.</p>",
+        "message": "<p>Measures the number of bytes that could be saved by compressing textual files. Some files listed below might not be compressed at all, some might be already compressed with Gzip but would become even lighter with Brotli.</p><p>All major server systems are now compatible with Brotli.</p><p>Note that compressing small files (< 1 KB) is arguable, and that some assets such as images should not be compressed as it is already included in their format. <a href=\"https://gist.github.com/gmetais/971ce13a1fbeebd88445\" target=\"_blank\">Here</a> is a list of Content-Types that should be compressed.</p>",
         "isOkThreshold": 20480,
         "isBadThreshold": 204800,
         "isAbnormalThreshold": 409600,