We want to encourage new users to use their real name as their user name when registering, and to discourage or prohibit them from using an email address or just a first name.  Where can we insert text to do so?  Can we place restrictions on allowable user names?

If I’m correct from v5 the only way to sign in will be by using emails due to security issues. If so, you should start transition from usernames to emails. I’m not sure if encouraging users to use they names it’s a good idea. I already started transition from usernames to emails… 

12 minutes ago, Edjazoli said:

If I’m correct from v5 the only way to sign in will be by using emails due to security issues. If so, you should start transition from usernames to emails. I’m not sure if encouraging users to use they names it’s a good idea. I already started transition from usernames to emails… 

The above would be relevant for logging in - but the user isn't talking about that, they are talking about discouraging the use of the email address as the display name.

22 hours ago, NSPN said:

Where can we insert text to do so?

Go to the following area in the ACP (Customization -> (Localization) Languages) and click 'Add phrase' and create an entry with username_desc as the key, and put your content into the default value field.

Result:

Where to go in the ACP:

 

22 hours ago, NSPN said:

Can we place restrictions on allowable user names?

 

Perfect answer.  Thanks for taking the time to explain so clearly.  You're an asset to the community.