ENH: allow Panel.shift on items axis

[mortada]

I'm not sure why Panel.shift currently raises with axis='items', while axis=0 works.

This PR enables axis='items' and also updates the docstring to show the parameter periods instead of the deprecated lags

[jreback]

looks fine. prob some leftover code :)

pls add a release note
if you don't see test for all axes, can you add them (to cover the bases).

[mortada]

• added a release note
• there were tests for major and minor, and I've added a test for items

[jreback]

aweseom. ping on green.

[mortada]

@jreback build is green now

[shoyer]

Thanks @mortada !