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)
diff --git a/src/version.c b/src/version.c
index 3f6b5ec..78baf47 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    697,
+/**/
     696,
 /**/
     695,