docs: through a mount a malformed write is EINVAL and one that fails is EIO, and the error words keep it so

The served README said every refused write reads as EINVAL, but a mount's
errno is 9ns's reading of the error's words, so a no-match or out-of-range
address and a refused Exit came back EIO, as the dogfood run saw. That split
is the useful one, malformed against failed, so the docs now state it rather
than the words being bent to EINVAL; a test pins each error's words to its
side of it. It caught one: 'regular expression search took too long' read as
ENAMETOOLONG, and now says it gave up past its step budget.

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

1 files changed, 1 insertions, 1 deletions

diff --git a/src/fs-help.txt b/src/fs-help.txt
index 689d8839..4633d91c 100644
--- a/src/fs-help.txt
+++ b/src/fs-help.txt
@@ -40,6 +40,6 @@ Pitfalls, one each:
   tag reads the path, then the tag's own text; writes append to that text; truncating clears it.
   A terminal's body is a history snapshot frozen per open; pty/data is the live stream.
   exec: a word no builtin knows is typed into a terminal in the pane's directory, and not an error.
-  A refused write reads as EINVAL through a mount: the log's err record says why.
+  Through a mount a malformed write is EINVAL, a well-formed one that fails EIO: log's err says why.
   lock/unlock on a pane's ctl need a held fd (exec 3>ctl); a lock someone holds fails: retry.
   Closing a column's last pane leaves a +New in its place; Delcol closes the column.