better doc python interface differences vs cpp, or create more examples

[cyrush]

A few cases:

Arg order in relay funcs isn't same as C++, tell folks to prefer named args in python

relay.io.load(node=n,path=p)

leave array vs leaf node acces:

n.fetch("child") vs n["child"] when child is a leaf

[cyrush]

Some of this now exists:
https://llnl-conduit.readthedocs.io/en/latest/tutorial_python_basics.html#differences-between-c-and-python-apis

Sufficient for 0.6.0.

But, we don't have docs for relay funcs that cover this, so I am not closing, instead moving to future release.