docs+comments: say what changed instead of tagging the tracker item

The prose added by this branch named the tracker item in seventeen
places, which check-issue-refs.sh rejects: a `#N` tag is dead weight for
anyone reading the public mirror, where no issue data exists. Each one
now states the fact it was pointing at — the parent field is gone — so
the sentence stands on its own.

Two of those lines also carried a rustdoc break: `[`write`]` in
topology.rs is ambiguous between the module's own `write` fn and the
`write!` macro, which `-D rustdoc::broken-intra-doc-links` fails. Spelled
`[`write()`]`, per rustdoc's own suggestion.

The host_config.rs rewrite is two lines rather than three so the doc
block stays under check-comment-blocks.sh's 30-line ceiling.

docs/process/conventions.md

@@ -89,8 +89,8 @@ angle-bracket and asterisk shapes below are structurally safe.
   inbox view; no agent ever `recv`'s them.
 
 `<parent>` and `<children>` were two more, resolved against a
-`topology.json` parent field. #4472 removed that field and both
-sentinels with it: address `operator` where you would have said
+`topology.json` parent field. That field and both sentinels no longer
+exist: address `operator` where you would have said
 `<parent>`, and name the recipients (or broadcast to `*`) where you
 would have said `<children>`. Nothing rewrites a recipient at send time
 any more — what an agent passes is what the broker stores.