Fix confusing inline help text in the menus editor

[DrewAPicture]

On the menus screen, the inline help text under the 'Menu Structure' section of the editor always implies that the "next step" is to add menu items, regardless of whether you've just created a new menu or opened an existing one. This is remnant of the "stepped" process of creating a new menu.

The text reads:

Next, add menu items (i.e. pages, links, categories) from the column on the left.

It should be rather:

Add menu items (i.e. pages, links, categories) from the column on the left.

Patch attached.

[SergeyBiryukov]

In 24185:

Update the instructions on Edit Menus screen. props DrewAPicture. fixes #24261.