patch 8.1.1942: shadow directory gets outdated when files are added

Problem:    Shadow directory gets outdated when files are added.
Solution:   Add the "shadowupdate" target and add a few comments.
diff --git a/src/version.c b/src/version.c
index e6f7d14..64735b2 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1942,
+/**/
     1941,
 /**/
     1940,