Navigation Block: Consider rewording placeholder instructional text

[getdave]

With the new features for creating Navigation blocks from Pages, Menus or Empty in #18869 there has been a lot of discussion about wording for the instructional text.

This Issue tracks the need to revisit the pieces of text once #18869 is merged.

This approach was suggested by @talldan and @karmatosed in their reviews.

Some useful links to discussions about alternative wording:

• Nav block: enable creation from existing WP Menus #18869 (comment)
• Nav block: enable creation from existing WP Menus #18869 (comment)

[talldan]

I left a few more comments than those mentioned above. The copy I felt could be improved:

• "Select where to start from…", doesn't seem like a very natural phrase. My suggestion was "Choose a starting point…", @paaljoachim also mentioned "Select an option…", which I think is better.
• "Create a navigation from all existing pages, or create empty.", 'create empty' sounded unusual to me, my suggestion was "Create an empty navigation or create from existing pages.".
• "Create a navigation from a menu or create empty.", similarly here my suggestion was "Create an empty navigation or use an existing menu."

[michelleweber]

• "Choose a starting point" or "Select an option" both sound smoother, but I would love for this line to be clearer about what I'm picking. Is it the location, or am I choosing whether this will use an existing menu? It's not clear from the text, and it's not clear from the options underneath, esp since the horizontal line makes me unsure if the stuff underneath is a totally separate set of options, or part of these options.
• For the second, I'd go with "Include all existing pages here, or add an empty Navigation block."
• And for the third, "Use an existing menu here, or add an empty Navigation block."