document that sync_imports() can't handle "import foo as bar"

[fperez]

Consider (this assumes a cluster is up and running):

from IPython.parallel import Client
rc = Client()
dv = rc[:]
with dv.sync_imports():
    import numpy as np

a user would assume then that np would be defined in the engines, but it's not.

[minrk]

as discussed on IRC, the as foo part of import statements appears to actually be handled entirely outside the import machinery, and thus not accessible to a clean import hook / override. Some ugly frame inspection or similar would be required, so we have no plans to fix this for the time being, other than saying "you can't do that :(".

[fperez]

I think we can close this issue once we add a simple .. warning:: block to the docs pointing this limitation out.

[fperez]

I've changed the label to simply docs since it's really a language limitation, not a bug on our side.