DOC: DataFrame Properties
Added explanations for `axes` and `shape`

could  you reconcile this with the same properties in `core/series.py`, & `core/generic.py` (we would normally use the `Appender` decorator, but I don't think it works on properties.

Just want to make sure that these descriptions are all consistent.
 Updated with changes.
 pls squash, otherwise looks good.
 can you rebase/squash. 
 merged via https://github.com/pydata/pandas/commit/873bbc2ada4f37a51a397cdb3bf3301e522dd236
