Not exactly what you ask, but I have added some text/description in this way: 

https://www.gulla.net/episerver-help-texts-improved/

If your entire manual/description is too large, you could just add a link this way. It will be more visible than a menu item on the help menu...

I've actually considered this byself, my solution was to follow https://www.jondjones.com/learn-episerver-cms/episerver-developers-tutorials/episerver-customizing-episervers-ui/displaying-a-custom-on-page-editing-view/ which allows you to add an extra edit view for documentation. Then when that view renders it shows the docuemntation for whatever block/page/media type you are on.

I ran into a simular issue since we have over 150 web authors, so we needed a centeralized place to put the help.   

I don't know if this will help you, but what I did was create a help sub-section within Epi and set the permissions to where only web editors can access it, and only when they are logged into Episerver.

I then added a link to the top nav so it has the highest amout of visabilty. 

This gives the admins the abilty to adjust the Episerver help page within Epi and things can be up to date.  We also use the help section to make announcments of new features and host our user group videos.  

Adding this link was really simple as well, though there are better ways to get the link than hard code it, but this is for internal use (you just need to make sure the permissions are set up correctly)

  [MenuProvider]
    public class CmsMenuProvider : IMenuProvider
    {

        public IEnumerable<MenuItem> GetMenuItems()
        {
            var menuItems = new List<MenuItem>();
            menuItems.Add(new UrlMenuItem("EPiServer Help",
                MenuPaths.Global + "/cms" + "/cmsMenuItem",
                UriSupport.ResolveUrlFromUIAsRelativeOrAbsolute("/help/"))
            {
                SortIndex = SortIndex.Last,
                IsAvailable = (request) => PrincipalInfo.HasEditorAccess
            });

            return menuItems;
        }