Make the `slices` and `slices_over` example more clear and that merge usually follows these operations.

[HGWright]

📚 Documentation

Users have reported confusion over what slices and slices_over actually do and which way around they are. It should also be in the docs that merge is the next step to combine all of these slices back together again.

Improving the example on the docs page would fix this, with a merge step after to recombine. And a comment on the docs about the difference between slices and slices_over and which way around they are would help to clear up that confusion.

[trexfeathers]

From @SciTools/peloton:

• We believe the examples themselves are good
• The main confusion is the naming of the functions, and this probably affects the confusing phrasing
• A note about merge being useful would be a valuable addition
• A workflow demonstrating slicing-then-merging would be ideal for a Diataxis how-to section, when we get to that (#6511)