Documentation: document platform specific -E commands (#10446)

Signed-off-by: Kovarththanan Rajaratnam <[email protected]>

Source/cmakemain.cxx

@@ -74,8 +74,10 @@ static const char * cmDocumentationOptions[][3] =
    "information. Commands available are: chdir, copy, copy_if_different "
    "copy_directory, compare_files, echo, echo_append, environment, "
    "make_directory, md5sum, remove_directory, remove, tar, time, "
-   "touch, touch_nocreate, write_regv, delete_regv, comspec, "
-   "create_symlink."},
+   "touch, touch_nocreate. In addition, some platform specific commands "
+   "are available. "
+   "Windows: comspec, delete_regv, write_regv. "
+   "UNIX: create_symlink."},
   {"-i", "Run in wizard mode.",
    "Wizard mode runs cmake interactively without a GUI.  The user is "
    "prompted to answer questions about the project configuration.  "