Warn when horizontal rule is used in v3

[jyasskin]

kramdown-rfc/lib/kramdown-rfc2629.rb

Lines 935 to 937 in 18d5a95

	def convert_hr(el, indent, opts) # misuse for page break
	"#{' '*indent}<t><vspace blankLines='999' /></t>\n"
	end

converts markdown ----- horizontal rules into <vspace> elements. xml2rfc warns (at least in the invocation used by i-d-template) when it sees <vspace>. It'd be better to warn that the input is invalid than to warn that the intermediate file is invalid.