Add nuance to join choice algorithm

[awoods187]

In our docs today we say By default, CockroachDB uses merge joins whenever possible because they are more performant than hash joins, computationally and in terms of memory. https://www.cockroachlabs.com/docs/dev/joins.html

Whenever possible is kind of misleading, I think it was supposed to say whenever the results are ordered for a merge. Technically merge join is always possible if you resort both sides but we don't do that

The sentence that follows implies that we can only do merge joins when both sides are already ordered, in which case "whenever possible" is correct. This was true with the HP, but this needs updating with more nuance.

[awoods187]

cc @RaduBerinde

[jseldess]

@awoods187, moving this to 19.2. Adding more work to the 19.1 scope should come with removing something as well. We have a lot on our plate.