patch 8.2.4978: no error if engine selection atom is not at the start

Problem:    No error if engine selection atom is not at the start.
Solution:   Give an error. (Christian Brabandt, closes #10439)
diff --git a/runtime/doc/pattern.txt b/runtime/doc/pattern.txt
index 7de1824..50fd53c 100644
--- a/runtime/doc/pattern.txt
+++ b/runtime/doc/pattern.txt
@@ -379,7 +379,7 @@
 1. An old, backtracking engine that supports everything.
 2. A new, NFA engine that works much faster on some patterns, possibly slower
    on some patterns.
-
+								 *E1281*
 Vim will automatically select the right engine for you.  However, if you run
 into a problem or want to specifically select one engine or the other, you can
 prepend one of the following to the pattern: