Clarify that SVG does not need to be valid XML

[real-or-random]

The conformance section says:
"Additional conformance classes depend on whether the content is also valid and well-formed XML":

svgwg/master/conform.html

Lines 550 to 551 in 51ff4ba

	Additional conformance classes depend on whether the content
	is also valid and well-formed XML [<a href="refs.html#ref-xml">xml</a>].

I think it should say:
"Additional conformance classes depend on whether the content is also well-formed XML".

None of the conformance classes require that the document is valid XML. (The only requirement that seems to be "imported" from XML is the validity of the id attribute.)

Moreover, none of the examples in the entire spec contain a <!DOCTYPE svg> declaration which would be required to be valid XML 1.0. (See XML 1.0 Section 2.8 "Prolog and Document Type Declaration", which says "The document type declaration MUST appear before the first element in the document.")

In general, I think it will be helpful to explain in more natural language that:

• SVG does not need to be valid XML. (Since SVG is defined in terms of XML, readers could reasonably expect that valid XML is required)
• A DOCTYPE declaration is not necessary nor recommended. (This was removed in 96e5189 for some reason.)