`docs/swarm/ca.md` said it under *Constraints on the material* and again
under *Distributing the root*; this module's header said it a third time,
directly above a line already pointing at that doc.
Measured: 6-gram intersection between the module's comments and the doc,
32 -> 6. The sentence now appears once in the tree.
Module header keeps what a reader editing this file cannot see from the
config: that one root makes cross-hive trust O(1), that the two
provisioning modes differ only in who writes the artifacts, and the ⚠️
that both root key and root cert are runtime files — the cert as a
consequence, and that one costs something real, since nothing whose trust
store is built at build time can name it. Everything behind those is a
`docs/x.md::Section` pointer with the heading checked to exist.
Dropped the O(n²)-per-peer-pinning story: the live reason is that one root
is O(1), and how it used to be configured is history.
In the doc, *Distributing the root* now links to *Constraints on the
material* rather than restating it. Intra-doc `#anchor` links are already
the convention there — 85 of them across `docs/`, 8 in `gateway.md` alone.
Comments + one doc paragraph; no behaviour change. Refs #3901.
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