Should update unkeyed join error message given `on`

[MichaelChirico]

The error message we get when we perform an unkeyed join now seems outdated:

When i is a data.table (or character vector), x must be keyed (i.e. sorted, and, marked as sorted) so data.table knows which columns to join to and take advantage of x being sorted. Call setkey(x,...) first, see ?setkey.

Since I solved my problem by simply adding on, and it seems that on is now generally the preferred approach for merges, should we not update this message?

[MichaelChirico]

How about:

When i is a data.table (or character vector), data.table must know to which columns to join; this requires that either on is specified (see ?data.table) or x is keyed (i.e. sorted, and, marked as sorted, e.g. with setkey); there may be speed advantages if x is keyed and thus sorted.

Could also perhaps add a link to #1232 to the message, on, setkey, or some combination.