DOC: Fixes after #17482
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Some doc fixes related to #17482:
- `func:` -> `:func:` 
    - See: https://github.com/pandas-dev/pandas/pull/17482#discussion_r139148962
- Added `cdate_range` to the public api so that it works in the docs
   - Ask for it to be added to docs:  https://github.com/pandas-dev/pandas/pull/17482#discussion_r138609092 
    - Public api issue pointed out: https://github.com/pandas-dev/pandas/pull/17482#discussion_r139148705
- Added `cdate_range` to `tests\api\test_api.py` to ensure that adding it to the top-level api is tested
- Removed the "experimental" from `cdate_range` docstring
    - See: https://github.com/pandas-dev/pandas/pull/17482#discussion_r138504193
- Fixed an error in the docstring for `interval_range` (strings aren't supported).