sort: promote '--debug'

* src/sort.c (usage): Mention --debug in KEYDEF paragraph.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/sort.c b/src/sort.c
index b1948f20b..399b9644d 100644
--- a/src/sort.c
+++ b/src/sort.c
@@ -518,7 +518,7 @@ effect, characters in a field are counted from the beginning of the preceding\n\
 whitespace.  OPTS is one or more single-letter ordering options [bdfgiMhnRrV],\
 \n\
 which override global ordering options for that key.  If no key is given, use\n\
-the entire line as the key.\n\
+the entire line as the key.  Use --debug to diagnose incorrect key usage.\n\
 \n\
 SIZE may be followed by the following multiplicative suffixes:\n\
 "), stdout);