fix: add usage for preserve comments

And remove the `-c` shortcut as it'll be a lesser used option

cli/options.js

@@ -33,7 +33,6 @@ function options(args) {
       o: 'videos',
       m: 'inlinemin',
       H: 'header',
-      c: 'preserve-comments',
     },
   });
 

docs/usage.txt

@@ -13,6 +13,7 @@
     --videos              encode videos (and their poster image) - disabled by default
     --nosvg               don't compress SVG (through SVGO)
     --skip-absolute-urls  don't inline links with absolute URLs
+    --preserve-comments   keeps HTML comments
     -V, --version         output the version number
     -h, --help            output usage information