doc(layout): Encourage use of username with set_user

[ketiltrout]

The previous layout docuemntation implied you needed to pass your "real name" (whatever that is) when calling set_user.
This is problematic for a few reasons:

• it confuses the notion of someone's actual name and the value of the CHIMEwiki user_real_name field (which is what's being matched here)
• the user_real_name field is typically populated by sysadmins when creating a wiki account. It's available and can be changed by users in the wiki profile page, but most wiki users don't change it nor do they even know what the value has been set to.
• There's absolutely no validation performed on the column. I've seen misspelled names, blank names, email addresses in this column.
• There's no requirement that the values of user_real_name be unique.

This changes the docs to encourage using CHIMEwiki usernames when calling set_user. Usernames are much better to use because:

• they're guaranteed to be unique
• they're unambiguous in form, meaning simple string matching is a reasonable thing to do.
• they're well known to users, since users use them to log into the wiki

Note: this doesn't change any functionality: users are still able to pass the value of the user_real_name field associated
with their account to this function.

(Bonus reading: Personal names are never a good thing to key a database on for multiple reasons)

[anjakefala]

Thanks for the documentation on the change!