I was looking to do the same thing, but none of it made any sense to me. So I did my own method, I made new pathing blockers and made their models look like trees (or whatever), now you can terrain like you normally would, but you wouldn't be able to go over them. I'll tell you how.
-----The Doodads-----
First: Terrain your wanted doodads as you normally would, making sure they are fairly close together (Your units can still go around them). Mix it up a little, make different plants, trees, rocks, ect. Change the Editor - Minimum Scale and Maximum scale to your preference, so there will be a vast selection of sizes.
-----THE BLOCKER------
First: Go to the Object Editor
Second: Destructables
Third: Pathing Blockers - Create a new one of the type you want (air, ground, both, make sure it is (large)) for each type of doodad you in your map (that you don't want your units to fly over/move through). It won't take that long once you have one already (CoPy & pAsTe FTW).
Fourth: Depending upon what you chose for your doodads, a tree, a rock, whatever. Change the Model File as well as the Variations of the new blocker to match your chosen doodads. Also depending on which doodads you chose, they will have a Replaceable Texture File and Replaceable Texture ID,
NOTE: FOR THIS TO WORK, match your new blocker to its respective doodad's Replaceable Texture File & ID.
Sixth: Match the size variations you want your new blocker to range from to your chosen doodad. (Editor - Minimum Scale/Maximum Scale).
Seventh IMPORTANT: Change the new blocker's Pathing - Pathing Texture to NONE for the moment.
-----PLACING YOUR NEW BLOCKERS-----
First: Leaving the Object Editor, go to Advanced up on the main window toolbar.
Second: Replace Doodads
Third: For the OLD DOODAD, select one of your chosen doodads. For the NEW DOODAD select your new blocker that matches that doodad. Click OK.
Fourth: Rinse & Repeat for all of the blockers you've made.
Fifth: Go back into Object Editor and change your blocker's Pathing - Pathing Texture back to its (type) (Large).
NOTE: If you didn't set your pathing to NONE before this step, then a message in your main WE window will say "Could not place in current positions... blah blah.
That's IT!!! Enjoy your new map that looks beautiful (maybe
and doesn't let units move through/over the terrain.