patch 8.1.0697: ":sign place" requires the buffer argument

Problem:    ":sign place" requires the buffer argument.
Solution:   Make the argument optional.  Also update the help and clean up the
            sign test. (Yegappan Lakshmanan, closes #3767)

src/version.c

@@ -799,6 +799,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    697,
 /**/
     696,
 /**/