patch 8.2.2805: Vim9: cannot use legacy syntax in Vim9 script

Problem:    Vim9: cannot use legacy syntax in Vim9 script.
Solution:   Add the :legacy command.
diff --git a/runtime/doc/vim9.txt b/runtime/doc/vim9.txt
index 0fc7bf5..0a2ea62 100644
--- a/runtime/doc/vim9.txt
+++ b/runtime/doc/vim9.txt
@@ -61,12 +61,17 @@
 rewrite old scripts, they keep working as before.  You may want to use a few
 `:def` functions for code that needs to be fast.
 
-						*:vim9* *:vim9cmd*
-:vim9[cmd] {cmd}
+:vim9[cmd] {cmd}				*:vim9* *:vim9cmd*
 		Execute {cmd} using Vim9 script syntax and semantics.
 		Useful when typing a command and in a legacy script or
 		function.
 
+:leg[acy] {cmd}					*:leg* *:legacy*
+		Execute {cmd} using legacy script syntax and semantics.  Only
+		useful in a Vim9 script or a :def function.
+		Note that {cmd} cannot use local variables, since it is parsed
+		with legacy expression syntax.
+
 ==============================================================================
 
 2. Differences from legacy Vim script			*vim9-differences*