Update docs to reference importing into an existing table

[jseldess]

We now have 19.2 docs on using IMPORT INTO to import into an existing table, but we still have statements throughout the docs that it's not possible to import into an existing table. We need to find and update all of these. We also need to update any reference to import best practices, for example, https://www.cockroachlabs.com/docs/dev/performance-best-practices-overview.html#bulk-insert-best-practices and https://www.cockroachlabs.com/docs/dev/sql-faqs.html#how-do-i-bulk-insert-data-into-cockroachdb.

@lhirata, if you already have an issue to polish our import into docs, feel free to close this.

[lnhsingh]

Background info if another writer picks this up:

@jseldess added in his review that we should be more opinionated on when to use IMPORT INTO vs. multi-row insert.

I asked @dt if he has any advice on when to use either. He said: "I don't have much in the way of comparative perf numbers but Roland might. IMPORT will take the table offline, which is one thing to keep in mind. If you're serving traffic at the same time, that might be a deal breaker. I would think that it is probably a lot faster for anything beyond like 100mb or some pretty small number. I'd guess anything past 100k rows but that is a completely uninformed guess."

I followed up with @rolandcrosby, who said he doesn't actually have any measurements on that and that he can look. (Need to follow up with him on this)

@bdarnell might have insight into when to use IMPORT INTO vs. multi-row insert.

[bdarnell]

It all comes down to whether you can tolerate taking the table offline for the duration of the import. If that's acceptable to you, IMPORT will be the fastest option. But I think that's rarely going to be the case, so I'd assume/recommend INSERT by default.