Update docs to include all shipping methods, fix typo on config shipping page

Add docstrings to the following classes in 'oscar.apps.shipping.methods':
* Free
* FixedPrice
* TaxExclusiveOfferDiscount
* TaxInclusiveOfferDiscount

Fix typo in 'docs/source/howto/how_to_configure_shipping.rst':
* Change 'shipping.addr' to 'shipping_addr'