Integer NA Doc Updates

[WillAyd]

With some of the work being done in #21160 do we want to update the documentation section on Integer NA support, or do we feel it is still too experimental at the moment?

http://pandas.pydata.org/pandas-docs/stable/gotchas.html#support-for-integer-na

[gfyoung]

Seems reasonable to do so. You can always qualify the documentation by stating that the behavior is not 100% stable (i.e. still somewhat experimental) if need be.

[jorisvandenbossche]

Yes, +1 on mentioning it there.

[PSY27]

i am will to do it