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.

hive-c0re/src/lifecycle/host_config.rs

@@ -92,8 +92,8 @@ fn config_bind_source(name: &str) -> PathBuf {
 /// directories so nspawn doesn't refuse to start; missing dirs are
 /// non-fatal.
 ///
-/// The only caller left is the `ManageRootAgent` grant — #4472 removed the
-/// parent/child tree that used to hand every agent its own children here.
+/// The only caller left is the `ManageRootAgent` grant; the parent/child
+/// tree that handed every agent its own children here is gone.
 ///
 /// **Three dirs, three different answers** — the uniformity of the
 /// original loop is what hid that:
@@ -335,8 +335,8 @@ async fn set_nspawn_flags(
     // now says so out loud instead of deriving it from a field that no
     // longer discriminates.
     //
-    // This is the *only* cross-agent mount left. #4472 dropped the
-    // topology parent field, and with it the unconditional grant every
+    // This is the *only* cross-agent mount left. Dropping the topology
+    // parent field took with it the unconditional grant every
     // agent used to get over its own direct children — so an agent with
     // no capability now sees its own dirs and nothing else.
     if crate::capabilities::has_cap(agent_name, Capability::ManageRootAgent) {