Complete over buffer anchors in adoc-goto-ref-label

[bbatsov]

Small ergonomics win: adoc-goto-ref-label (C-c C-a) used to make you type the anchor id from memory. Now it offers the buffer's anchors via completing-read, reusing the adoc--collect-anchor-ids collector from the completion work. It stays permissive (no require-match), so an id that isn't defined yet can still be entered, and the cross-reference at point is still the default.

This came out of a quick survey of which commands read from the minibuffer - adoc-goto-ref-label was the one clear candidate (the only other reads are adoc-insert-link's URL/text, which have no good completion source).

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