patch 8.2.2511: Vim9: cannot use Vim9 script syntax in some places

Problem:    Vim9: cannot use Vim9 script syntax in some places.
Solution:   Add the :vim9cmd command modifier. Incompatible: Makes ":vim9"
            mean ":vim9cmd" instead of ":vim9script".

runtime/doc/vim9.txt

@@ -51,6 +51,7 @@ The Vim9 script syntax and semantics are used in:
 - a function defined with the `:def` command
 - a script file where the first command is `vim9script`
 - an autocommand defined in the context of the above
+- a command prefixed with the `vim9cmd` command modifier
 
 When using `:function` in a Vim9 script file the legacy syntax is used, with
 the highest |scriptversion|.  However, this can be confusing and is therefore
@@ -60,6 +61,12 @@ Vim9 script and legacy Vim script can be mixed.  There is no requirement to
 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}
+		Execute {cmd} using Vim9 script syntax and semantics.
+		Useful when typing a command and in a legacy script or
+		function.
+
 ==============================================================================
 
 2. Differences from legacy Vim script			*vim9-differences*