docs: rewrite the remaining 3 obvious-framing hits too

Not condescending (they're negated -- non-obvious/not obvious is the
opposite claim, not the flagged pattern) but the negated-obvious
phrasing itself is worth a direct rewrite regardless of what triggered
the review: integrations/matrix.md (non-obvious -> subtle),
process/conventions.md (if non-obvious -> when the reasoning isn't
self-evident), swarm/ca.md (not obvious -> subtle).

docs/integrations/matrix.md

@@ -268,7 +268,7 @@ Both fixed in `nix/host-modules/hive-matrix.nix` via two derivations:
   incremental cost) and (b) installs `fluffychat-web-imaging`'s
   outputs into `$out`.
 
-Two non-obvious details worth knowing before touching either derivation:
+Two subtle details worth knowing before touching either derivation:
 
 - **`make -C js`** instead of `cd js; make` — keeps the build-phase
   pwd at the source root so `installPhase` doesn't have to know

docs/process/conventions.md

@@ -439,8 +439,8 @@ POST handlers both call into them so the two surfaces never drift.
 ## Commit messages
 
 Short, lowercase, no `Co-Authored-By` trailer. Imperative mood, no
-period. Body explains *why* if non-obvious; otherwise the subject
-alone is fine. Wrap at ~72 cols.
+period. Body explains *why* when the reasoning isn't self-evident;
+otherwise the subject alone is fine. Wrap at ~72 cols.
 
 ## Commit before test
 

docs/swarm/ca.md

@@ -159,7 +159,7 @@ The root **key** is a runtime file for the reason in
 
 The root **certificate** is a runtime file as a _consequence_ — it lives
 beside the key under `swarm.ca.stateDir` — and that has a cost worth
-naming, because it is not obvious and it bites at a distance:
+naming, because it is subtle and it bites at a distance:
 
 > **Nothing whose trust store is assembled at build time can reference
 > the swarm root.** `security.pki.certificateFiles` is read inside the