patch 8.1.1554: docs and tests for :const can be improved

Problem:    Docs and tests for :const can be improved.
Solution:   Improve documentation, add a few more tests. (Ryuichi Hayashida,
            closes #4551)
diff --git a/src/version.c b/src/version.c
index aa11b7e..fcdf562 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1554,
+/**/
     1553,
 /**/
     1552,