flag: clarify comment to avoid shell syntax confusion

Updates #22961

Change-Id: Ib2f41aefb4f6470598d8637611da5491156ea840
Reviewed-on: https://go-review.googlesource.com/82015Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/flag/flag.go

@@ -43,8 +43,9 @@
 	The last form is not permitted for boolean flags because the
 	meaning of the command
 		cmd -x *
-	will change if there is a file called 0, false, etc.  You must
-	use the -flag=false form to turn off a boolean flag.
+	where * is a Unix shell wildcard, will change if there is a file
+	called 0, false, etc. You must use the -flag=false form to turn
+	off a boolean flag.
 
 	Flag parsing stops just before the first non-flag argument
 	("-" is a non-flag argument) or after the terminator "--".