Let the tag's undo bring back what a 9P truncate cleared

Truncating the tag file replaced its text without an undo point, so the
cleared text was gone for good. It is recorded now like a typed edit, the
history is kept, and docs/fs.md says so.

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 c43034f6..589f4287 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -185,7 +185,9 @@ pane.
 `tag` reads the whole tag as the pane shows it: the computed path, dirty
 marker or PDF page, then the text you may edit. A write appends to that text,
 newlines included, and a tag with more than one line takes a row per line on
-screen; truncating `tag` clears it, as acme's `cleartag` does.
+screen; truncating `tag` clears it, as acme's `cleartag` does. The clearing
+is an edit of the tag like a typed one and its undo history is kept: `u` in
+the tag brings back the text it cleared.
 
 Stats report real lengths for `index`, `status`, `look`, `exec`, `listeners`,
 `name`, `body`, `tag`, `sel`, `ctl`, the range files and the flag files, and