Fix mergelist.Rd documentation

[trobx]

closes #7190
Another go at this - replaces a previous PR now deleted.

[trobx]

I hope this is helpful. Adding some explanation. Recall that the main issue is the fact that the current definition of join-from/-to incorrectly talks about row and column order.

• A table makes things much shorter and clearer: a user can see the pattern at a glance.
• Jan, you said wanted to abstract from row order, but I've nevertheless suggested including a column that states the output key in each case (but without further text). I think this is helpful information, and there is no risk of it confusing matters re. the join-from/to distinction - in fact a separate column makes it clear that row order is not part of that concept.
• Because the table is so clear I don't think we need a compare/contrast with x[i] to add clarity, which Michael had raised last time. Likewise I don't think there needs to be a potted verbal description of join-from/join-to now (I had something about the "searching"/"searched" tables previously) - we can just cut to the chase.

Please note two points about content:

• Because join-from/-to don't have any significance for the cross join, I've proposed listing them as "n/a" for that case (instead of "both" as in the current text). I actually think "both" might be a little confusing here.
• I've proposed leaving out the sentence about not using mult="error" to check for duplicates as somehow it seems to me an odd thing to warn about.