[bug] Documentation lists feature not released yet

[Irina Armean]

Firstly, thank you for your work on this very useful library!

• Did you find a bug in datatable, or maybe the bug found you?
  I was trying to use a function listed in the documentation only to noticed that it's marked as "Added in version 1.1.0" however the version 1.1.0 is not yet out.
  https://datatable.readthedocs.io/en/latest/api/dt/nunique.html

• How to reproduce the bug?
  Link to the documentation referencing 1.1.0 : https://datatable.readthedocs.io/en/latest/api/dt/nunique.html
  Git repo has 1.0.0 as latest release.

• What was the expected behavior?
  I would expect the documentation to be in line with the latest released version. If a "work in progress" release version is referenced then I would at the minimum expect an additional note to when that version will be released or is expected to be released.

• Your environment?
  What is your datatable version, python version, and operating system?
  datatable 1.0.0, python 3.8, Linux

• Tag the issue with [bug] or [segfault] (depending on whether it crashes
  Python or not).
  [bug]

• Thank you for contributing, and sorry for the inconvenience.