DOC: Update roadmap for completions
Moves "Improved documentation" to completed section.

I don't think any others can be marked as done yet, though "Numba-accelerated operations" may be getting close (is that right @mroeschke?).
Regarding Numba, pretty much complete in reference to the `and in groupby and window contexts` line in that section.
Will need to see if it's still relevant for the non groupby and window contexts ci/../doc/source/development/roadmap.rst:193:Heading capitalization formatted incorrectly. Please correctly capitalize "Completed Items" to "Completed items" 
 Fixed. great