Menu description administration cleanup

Removing two parts, "Menus are a collection of links used to navigate a website." - not sure if we should describe what menus are, seems like a rather edge case that they don't know this and are unable to understand this from the labels and links corresponding to something already on there site.

Then "If the Block module is enabled, each menu has a corresponding block that is managed on the blocks administration page." I shortened this to "Each menu has a corresponding block that is managed on the blocks administration page.". since, we can do the if for the user.

In the usability tests came forward that people have difficulty connecting the relationship between blocks and menus, as expected they didn't extract this from the description. Hence by just putting that up, it should be more scannable for those who need to make the relationship.

Also removed "Select an existing menu from this list to manage its menu links." explaining functionality "list links" or the title, which should be rather self explanatory.