A pattern anchoring some alternatives with ^ and not others is refused with why, not a bare bad regular expression

`^def|x` and `foo$|^zèbre` were refused with a bare "bad regular expression",
the same words as a syntax error, so a script could not tell what to change.
They now say "an alternation anchors every branch with ^ or none (^a|^b, not
^a|b)", through addr and Edit alike. fs.md states the rule and that a `$`
does not count toward it (`foo$|bar` is fine).

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

1 files changed, 3 insertions, 2 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 2a618173..58ba0539 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -459,8 +459,9 @@ The same code (`src/regexp.zig`) serves addresses, Edit, and normal mode's
   submatches, so Edit's `s` has no `\1`-`\9`.
 - A pattern holding `\n` runs over the whole text: there `^` may only come
   first and `$` only just before a `\n`, else it is refused.
-- An alternation must anchor every branch or none (`^def|^ ` works,
-  `^def|x` is refused).
+- An alternation must anchor every branch with `^` or none (`^def|^ `
+  works, `^def|x` is refused: `an alternation anchors every branch with ^
+  or none`). A `$` does not count: `foo$|bar` is fine.
 - A class may hold non-ASCII runes (`[éa-z]`, a range up to 256 runes); a
   wider range or a negated class with one (`[^é]`) is refused.
 - At most 512 operations (about 512 characters, counted after that