docs: correct six claims the code had outgrown

[haribo]

Summary

#662's five stale claims, plus #661's missing constraint — one PR because they touch the same
two files for the same reason: key is documented in the README and in the inventory host
table, and #661 itself says so ("…and the host table in docs/design/inventory.md if that one
is corrected under its own issue").

Each re-verified against develop before being changed:

where	said	now
docs/language.md:3-5	"One shipped construct has no chapter yet — as <user>"	names all five: as <user>, on, parallel, override def, the inventory syntax
docs/language.md:28	a def translates a shell result "via when/tags"	when is not a keyword — zero occurrences in internal/lang; the sentence now says what actually happens
docs/design/inventory.md:16-19	address | yes; no local, no interpreter	address required unless local, and the table carries local, interpreter and the free-form-variable row
internal/inventory/inventory.go:1-3	"no business variables … a parsed shellf-language inventory comes later"	both clauses were false — Vars is a field, and lang.ParseInventory shipped
internal/std/postgres/postgres.shellf:1-7	"Only these two", arguing a postgres.config would inherit an unresolved problem	four defs, and the argument recorded as a prediction the code overtook — a hundred lines above the postgres.config that resolves it
README.md:78, :115	key: as an ordinary override	says it must be unencrypted, since shellf parses the file and has no passphrase prompt (ADR-0026 §3)

Found while fixing it

README.md:115 said "Only address is required" three lines above "host self = { local: "true" } (no address needed)" — the same contradiction as the design doc's table, in the file
that had already been reviewed twice today. Corrected in the same sentence.

Not changed, deliberately

docs/dogfood.md:99 — "A plan cannot read another host's inventory entry" — is superseded by
ADR-0054 and stays as written. That file is a dated report of what a deployment could not
express, and history is allowed to describe a world the code has left (ADR-0001). It gains a
forward pointer saying where the finding went, which is what #662 asked for.

Test plan

Doc and comment changes only. Each validation criterion from the issue, checked:

via `when` in language.md                → 0
language.md names override def           → 1
inventory.md lists local / interpreter   → 1 / 1
postgres def count vs its header         → 4 / "Four defs"
"no business variables"                  → 0
README says "unencrypted"                → 2
every new doc link resolves              → 9 ok, 0 missing

go test ./..., test/lint.sh, test/changelog-rule.sh, test/changelog-frozen.sh green.

Closes #661
Closes #662