UPDATE FROM syntax

[ericharmeling]

Fixes #8219.
Documents cockroachdb/cockroach#39202.

Note that this PR will conflict with the updates to UPDATE documentation in #8656.

[cockroach-teamcity]

This change is 

[cockroach-teamcity]

Online preview: http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/69a27cd7da5b0b55164456b5c3e13d9b4bbab1ad/

Edited pages:

• v19.2/update.md
• v20.1/update.md
• v20.2/update.md

[RaduBerinde]

We are missing a description of the semantics of UPDATE FROM. Postgres docs have this note (which applies to us as well):

When a FROM clause is present, what essentially happens is that the target table is joined to the tables mentioned in the from_item list, and each output row of the join represents an update operation for the target table. When using FROM you should ensure that the join produces at most one output row for each row to be modified. In other words, a target row shouldn't join to more than one row from the other table(s). If it does, then only one of the join rows will be used to update the target row, but which one will be used is not readily predictable.

Because of this indeterminacy, referencing other tables only within sub-selects is safer, though often harder to read and slower than using a join.

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @RaduBerinde)

[ericharmeling]

TFTR, @RaduBerinde

I added a section that summarizes the Postgres docs. Please re-review for accuracy.

Reviewable status: complete! 0 of 0 LGTMs obtained

[cockroach-teamcity]

Online preview: http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/49ca64e010524bbd99184cd6b9e85454df7f653f/

Edited pages:

• v19.2/update.md
• v20.1/update.md
• v20.2/update.md

[RaduBerinde]

Reviewable status: complete! 1 of 0 LGTMs obtained

[lnhsingh]

Reviewable status: complete! 1 of 0 LGTMs obtained (and 1 stale) (waiting on @lnhsingh)