Joincol with no column to the right and Theme with no such theme say so

Both did nothing without a word, from a click or a ctl write alike, and a
script could not tell a no-op from success. Joincol in the rightmost column
now fails 'Joincol: no column to the right'. A Theme name that is none says so
and names the themes sharing its first letter, since all of them (ThemeSel
lists them) are too many for a message or a refused write; a startup config
line still fails quietly, as its other bad lines do.

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 4673e042..41b960f2 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -146,7 +146,9 @@ in Plan 9's words for a ctl (kernel/misc/parse.c:82-97), quoting the line:
 `unknown control message "X"`; `wrong #args in control message "X"` for an
 argument to a builtin that takes none, or none to one that needs it (`Msg`,
 `Mount`, `Find`, a setting's value but a switch's, which flips bare);
-`bad value in control message "X"` for a setting's value it does not take;
+`bad value in control message "X"` for a setting's value it does not take
+(for `Theme`, naming the themes that share the name's first letter, since
+all of them, `ThemeSel`'s list, are too many for an error);
 and `not a session control message "X"` or `not a window control message
 "X"` for a word of the other ctl. 9ns maps them all to EINVAL, and a write
 refused here has done nothing. A line that then fails as it runs fails the