Add version info for new columns

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Add a note that the Time Estimate and Time Spent export fields were only added in v10.0.

Why is this MR needed?

https://gitlab.zendesk.com/agent/tickets/82651.

Customer(s) on older versions lodged support request about "missing" fields on csv export after reading about these fields in our docs.