docs: explain external I/O fencing architecture #1160

Job	Run time
Validate (comment headers + format + tidy + commit msg)	35s
Linux --disable-cluster regression	-1s
macOS build-only (compile guard)	-1s
Linux enable-cluster (unit + regress + smoke TAP)	-1s
Security (cppcheck + scan-build)	-1s
	31s

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