add note about possible validation of xref id
add note about possible validation of xref id

anthonny/asciidoctor,benignbala/asciidoctor,gjtorikian/asciidoctor,cuit-zhaxin/asciidoctor,cuit-zhaxin/asciidoctor,glamouracademy/asciidoctor,asciidocfx/asciidoctor,neontapir/asciidoctor,anthonny/asciidoctor,benignbala/asciidoctor,yukisan/asciidoctor,gjtorikian/asciidoctor,neontapir/asciidoctor,glamouracademy/asciidoctor,asciidocfx/asciidoctor,pepijnve/asciidoctor,pepijnve/asciidoctor,yukisan/asciidoctor