Add a mention and/or example of .custom-select-sm to Custom Select section

Created by: tiesont

This would be a trivial update, but: can the Select Menu section of the custom form controls be updated to include a mention (or example) showing that there is a sizing class for a smaller version? My last attempt to issue a pull-request against the docs directory was auto-closed, or I'd just add it myself...

I could not find a mention of .custom-select-sm anywhere on that page, so I had to check the bootstrap.css file to find that there was, in fact, a sizing option.

On a related tangent: is there a reason not to also have a -lg version, or am I simply missing something?