Fix comment and add another comment (#148)

This comment seems to got wrong over time:

The config made sense first: df6286b2e9 (diff-e45e45baeda1c1e73482975a664062aa56f20c03dd9d64a827aba57775bed0d3)

Then the config became obsolete: 4e258087dc

Then the config became a (I think wrong) comment: 0167334910 (diff-e45e45baeda1c1e73482975a664062aa56f20c03dd9d64a827aba57775bed0d3)

I don't use Mac so I don't know but maybe the whole comment could be removed completely since the `cached` option was introduced.

docker-compose.override.yml

@@ -4,10 +4,12 @@ version: "3.4"
 services:
   php:
     volumes:
+      # The "cached" option has no effect on Linux but improves performance on Mac
       - ./:/srv/app:rw,cached
       - ./docker/php/conf.d/symfony.dev.ini:/usr/local/etc/php/conf.d/symfony.ini
-      # If you develop on Linux, comment out the following volumes to just use bind-mounted project directory from host
+      # If you develop on Mac you can remove the var/ directory from the bind-mount
-      # - ./var:/srv/app/var:rw
+      # for better performance by enabling the next line 
+      # - /srv/app/var
     environment:
       APP_ENV: dev