DOC: Remove old warning from dsintro.rst
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

The warning is about something that have been fixed for almost 3 years. Every time a new user excited about pandas start reading the docs, they have to waste brain-cycles ignoring that big red warning bubble.

Yes, good idea! Thanks
 actually could prob take out the Index warning as well (indexing.rst). Maybe also look for _older_ things like this elsewhere.
 Thanks for the quick merge @jorisvandenbossche . This certainly encourages me to keep sending patches like this. :)

@jreback, absolutely. This is my first touch in the code base, so I won't take on that endeavor. But if this inspires a maintainer to do it. I would be ever so glad. :)
 Yes, I think we should do a general search on version notices below 0.15 or so (also for notices that something was added in eg version 0.14 -> although maybe we could leave that for the 1.0 docs to do a big cleanup)
 Also removing the "experimental" tags on some sections if they're ready
 Regarding experimental, there is an issue for that: https://github.com/pydata/pandas/issues/5049
