in the next release

Which @donho just released a few mintues ago (thanks!). So now my official docs link points to the explanation with more detail.

I cannot find any instructions that say how to navigate submenus

The official docs say,

To add a built-in command, you need to provide the main menu name (as it appears in the main menu bar) as the value of the MenuEntryName attribute and the command’s item name (as it appears in the menu) as the value of the MenuItemName attribute. The MenuEntryName attribute must reference an entry on the main menu bar and must be an ancestor of the MenuItemName attribute, regardless of its depth.

Thus, MenuEntryName must always be a top-level (“from the main menu”), and MenyItemName is the name of the action any number of levels deep (“regardless of its depth”).under that top-level menu name.

Further, as will be explained better in the next release of the usermanual, you can also refer to any builtin menu entry through its command ID rather than the menu name

<Item FolderName="Sort Lines" MenuEntryName="Edit" SubMenuEntryName="Line Operations" MenuItemName="Sort Lines Lexicographically Ascending" />
<Item FolderName="Sort Lines" MenuEntryName="Edit" SubMenuEntryName="Line Operations" MenuItemName="Sort Lines Lex. Ascending Ignoring Case" />
<Item FolderName="Sort Lines" MenuEntryName="Edit" SubMenuEntryName="Line Operations" MenuItemName="Sort Lines Lexicographically Descending" />
<Item FolderName="Sort Lines" MenuEntryName="Edit" SubMenuEntryName="Line Operations" MenuItemName="Sort Lines Lex. Descending Ignoring Case" />

<Item MenuEntryName="Edit" SubMenuEntryName="Line Operations" MenuItemName="Sort Lines Lexicographically Ascending" />