A root exec or look that fails logs its err against the pane it ran at

`err - exec: ...` said nothing of where the line ran; the root's exec and
look run at the keyboard's pane, and their err now names it, as a click
there would.

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 7baac666..36fc7d7a 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -748,7 +748,9 @@ between words, and ends in an ellipsis, `…`, as an `err`'s reason is past
 200; its serial is the pane it ran at, `-` when the keyboard was on a
 column or workspace tag, or the line came to the root's ctl, `/tagexec` or
 a column's ctl or exec), and `err <serial|->
-<file>: <why>` for every write or truncation the tree refused or that
+<file>: <why>` (the serial the pane's whose file it is, and for the root's
+`exec` and `look` the pane the line ran at, `err 3 exec: ...`; `-` for
+another root file) for every write or truncation the tree refused or that
 failed -- through a mount a shell sees only the errno its kernel mapped the
 reply to, usually `Invalid argument`, and this is the reason (`err 3 addr:
 no match for regexp`). A record said again word for word, straight after