docs(docshost): name the bound-port cell's process-per-test contract

The cell writes the process-global BOUND_DOCS_PORT, so it is sound only under
a process-per-test runner. Say which runner that is rather than leaving it to
be re-derived: nextest, which both ci.yml's unit leg and the golden workflow
use. A threaded runner (bare cargo test) is out of contract for this cell,
because siblings would share the global.

The nuance is kept rather than flattened: the cell leaves BOUND_DOCS_PORT as
it found it, since the guard's Drop restores the 0 default, so it would not
poison a sibling even under a threaded runner. What breaks there is the cell's
OWN None-before assert, which any future reader or writer of that global could
invalidate by running first. Naming the failure precisely is the difference
between a comment that warns and one that can be checked.

Comment-only: every changed line, added and removed, is a comment. No
assertion, no code, no tag touched.

Co-authored by: hertz
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GY7rT7iPXEDNb7FN18WX1K