docs: data's truncation is pardes's own, a second write inserts, and a waiting pty/run is cancelled

Review asked the docs to say what a script trips on: acme ignores OTRUNC on
data (fsys.c:543), so truncating it is an extension; addr sits past each write,
so a second echo x > data inserts after the first; and a pty/run line waiting
for a fresh terminal's first prompt waits for ever if none is drawn, so the way
out is to interrupt the read. The lock's retry advice was already there.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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