avro

AVRO-3212: fix specification to allow doc in fixed types (#1342)

The currect specification omits the doc property on fixed types, even
though the implementation allows for it.
The spec is also wrong in the number of recognized properties on
records and fixed types - exact counts have been replaced with
"the following" to be more future proof (this is the wording
used for enums already, for example).
also aligned formatting for "namespace" attribute with the other
attributs for record and fixed