Edit's refusals name the pattern, not the line, and say when dot is empty

A ctl Edit that missed answered `Edit: no match for regexp "Edit
/nomatch/d"`, quoting the whole program; it now names the address that
missed (`no match for regexp /nomatch/`) and quotes no line after. An s
on an empty dot says `no substitution: dot is empty`. A \\1 is blamed
only once its pattern has matched, as before. tags.md says & is special
in s and not in c, a or i, as in sam.

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

1 files changed, 5 insertions, 1 deletions

diff --git a/docs/tags.md b/docs/tags.md
index c1034cb4..bfca4f0e 100644
--- a/docs/tags.md
+++ b/docs/tags.md
@@ -62,7 +62,11 @@ if every command ran; an error says why in acme's words and changes
 nothing. `p` and `=` print to the directory's `+Errors`. Left out: the file
 commands `b B D e r w f X Y`, the pipes `< | >`, and `\1`-`\9` in `s`,
 since mvzr keeps no submatches; acme applies changes that come out of
-sequence with a warning, pardes refuses the Edit.
+sequence with a warning, pardes refuses the Edit. In `s`, `&` in the
+replacement is the matched text (`\&` a plain `&`); in `c`, `a` and `i` an
+`&` is only an `&`, as in sam. A pattern that finds nothing says so with the
+pattern (`no match for regexp /nomatch/`), and an `s` on an empty dot says
+`no substitution: dot is empty`.
 
 `Undo` and `Redo` are acme's: typed or clicked in a pane's tag, or written
 to its `ctl`, they step the body back and forward through its edits, as the