How to create multi-value entries for e.g. TEL?

[htgoebel]

Thanks for improving the documentation about multi-value entries (see #78, #79). When rethinking this, I found a few more places for improvement:

1. How to create multi-value entries

The example from the documentation only shows a simple entry. This is an example for adding a second entry:

>>> import vobject
>>> j = vobject.vCard()
>>> e1 = j.add('email')
>>> j.email.value = 'jeffrey@osafoundation.org'  # intentionally using a different way here
>>> e2 = j.add('email')
>>> e2.value = "john@example.com"
>>> e1
<EMAIL{}jeffrey@osafoundation.org>
>>> e2
<EMAIL{}hartmut@goe-con.de>
>>> j.email
<EMAIL{}jeffrey@osafoundation.org>

2. How to determine if an entry is multi-valued?

According to this test, it seems that email entries (and presumable tel etc.) are always lists:

>>> k = vobject.vCard()
>>> k.add('email')
<EMAIL{}>
>>> k.contents['email'].__class__
<class 'list'>

Please documents this and also which entries are lists.

[wpercy]

Feel free to make a Pull Request with any changes you think would be helpful.