Document Memcache usage and configuration in settings files

IMO our documentation should be a bit more vague, since this highly depends on the website (traffic and used modules) and on the uses memcache settings (memory).

What about just putting some cache bins in memcache by default and allowing to add more. Currently we exclude form cache (good) but in some cases container / config / bootstrap could also be problematic.

Something like this might be better?

  // Define which cache bins should be stored in memcache. 
  // Some (highly volatile) bins could cause problems when stored in memcache, so by default we only add the largest ones in memcache. 
  // For example form cache should not be stored in memcache. 
  // Also bins like config / container / discovery could cause problems depending on your website / hosting. 
  $settings['cache']['bins']['page'] = 'cache.backend.memcache';
  $settings['cache']['bins']['dynamic_page_cache'] = 'cache.backend.memcache';
  $settings['cache']['bins']['render'] = 'cache.backend.memcache';
  $settings['cache']['bins']['menu'] = 'cache.backend.memcache';
  // You can manually add more cache bins here to memcache. 
  // This should be decided on a per site basis, and is also dependant on your memcache setup (amount of memory for ex.)
  # $settings['cache']['bins']['migrate'] = 'cache.backend.memcache';
  # $settings['cache']['bins']['geocoder'] = 'cache.backend.memcache';
  # $settings['cache']['bins']['file_mdm'] = 'cache.backend.memcache';
  # $settings['cache']['bins']['discovery_migration'] = 'cache.backend.memcache';
  # $settings['cache']['bins']['advagg'] = 'cache.backend.memcache';

  // Form cache should never be stored in memcache.
  $settings['cache']['bins']['form'] = 'cache.backend.database';

Changing default memcache setup will be discussed in #3334099: Improve memcache defaults and is out of scope here. I've added some remarks for the proposed comments in the MR.

Thanks sdstyles. This looks much better to me.