[bug] Documentation lists feature not released yet

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.