add clarity to document composition and XML excerpts

Section 0.1.1 has been modified as follows:

From:
> The XML notation used by the specification is formally defined by the AMQP DTD included in the specification.

To:
> "The authoritative form of the AMQP specification consists of a set of XML source documents. These documents are transformed into PDF and HTML representations for readability. The machine readable version of the AMQP DTD describes the XML used for the authoritative source documents. This DTD includes the definition of the syntax used in the excerpts of XML presented in the PDF and HTML representations."