patch 8.2.1342: Vim9: accidentally using "t" gives a confusing error

Problem:    Vim9: accidentally using "x" gives a confusing error.
Solution:   Disallow using ":t" in Vim9 script. (issue #6399)
diff --git a/runtime/doc/vim9.txt b/runtime/doc/vim9.txt
index 5d264b6..2c4d1db 100644
--- a/runtime/doc/vim9.txt
+++ b/runtime/doc/vim9.txt
@@ -190,8 +190,8 @@
 
 An existing variable cannot be assigned to with `:let`, since that implies a
 declaration.  Global, window, tab, buffer and Vim variables can only be used
-without `:let`, because they are are not really declared, they can also be
-deleted with `:unlet`.
+without `:let`, because they are not really declared, they can also be deleted
+with `:unlet`.
 
 Variables cannot shadow previously defined variables.
 Variables may shadow Ex commands, rename the variable if needed.
@@ -352,10 +352,11 @@
 |curly-braces-names| cannot be used.
 
 
-No :xit, :append, :change or :insert ~
+No :xit, :t, :append, :change or :insert ~
 
-These commands are too easily confused with local variable names.  Instead of
-`:x` or `:xit` you can use `:exit`.
+These commands are too easily confused with local variable names.
+Instead of `:x` or `:xit` you can use `:exit`.
+Instead of `:t` you can use `:copy`.
 
 
 Comparators ~