Adds comments about z-index for input groups and custom selects.
Adds comments about z-index for input groups and custom selects.

kurtrwall/cfgov-refresh,kurtrwall/cfgov-refresh,jimmynotjim/cfgov-refresh,imuchnik/cfgov-refresh,kurtw/cfgov-refresh,kurtw/cfgov-refresh,kurtrwall/cfgov-refresh,imuchnik/cfgov-refresh,imuchnik/cfgov-refresh,imuchnik/cfgov-refresh,kurtw/cfgov-refresh,jimmynotjim/cfgov-refresh,jimmynotjim/cfgov-refresh