stringsAsFactors=.2 is undocumented

[MichaelChirico]

Follow-up to #2025 -- we mentioned the change in NEWS but nothing in fread.Rd

[markderry]

I have updated the documentation and I am wondering if the following is still required for a PR?

• A new test
• A news entry

[MichaelChirico]

no test, but you can add a NEWS entry and please cite yourself for the fix. thanks!

[mattdowle]

Closed by Mark's #4745. Thanks!