A tag write refuses NUL and the other control characters; a Dump that fails says so

A NUL written into a tag went into the dump, which the host then could
not write, and Dump said nothing: a success that wrote nothing. Tag
writes, the workspace's, a column's and a pane's, refuse control
characters but tab and newline, DEL, C1 and bytes not UTF-8, EINVAL; and
Dump reports any failure of its own instead of swallowing it.

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

1 files changed, 3 insertions, 1 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 184f6131..95c72baa 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -130,7 +130,9 @@ Existing Plan9port/v9fs clients need a userspace bridge for QUIC.
              <serial>: acme's activecol, which the keyboard leaving for another column's
              tag does not move, so the two can differ -- the active column, where
              pane/new and a look place a pane next (- when there is none)
-/tag         the workspace tag; > replaces it, >> appends, one line
+/tag         the workspace tag; > replaces it, >> appends, one line; a control character
+             but a tab (a NUL too), DEL, a C1 control or bytes not UTF-8 are refused, `invalid
+             tag text: ...` (EINVAL), in any tag, a pane's or a column's too
 /tagexec     write a word: a middle click on it in the workspace tag; read as /exec. A
              pane's word (Undo, Msg, Save, Edit too) is refused there and at a column's exec,
              `not a session control message "Undo": write it to pane/<n>/ctl` (EINVAL),