patch 8.2.0491: cannot recognize a <script> mapping using maparg()

Problem:    Cannot recognize a <script> mapping using maparg().
Solution:   Add the "script" key. (closes #5873)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 45ef83a..74c3d1c 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -6824,6 +6824,7 @@
 		  "rhs"	     The {rhs} of the mapping as typed.
 		  "silent"   1 for a |:map-silent| mapping, else 0.
 		  "noremap"  1 if the {rhs} of the mapping is not remappable.
+		  "script"   1 if mapping was defined with <script>.
 		  "expr"     1 for an expression mapping (|:map-<expr>|).
 		  "buffer"   1 for a buffer local mapping (|:map-local|).
 		  "mode"     Modes for which the mapping is defined. In