Remove limitation about on conflict and returning

[jseldess]

Fixes #1693

[cockroach-teamcity]

This change is 

[cockroach-teamcity]

http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/4a64f876669eb0ab24ecbde961435fd4cc26e3ec/

[jseldess]

@couchand, this just involved removing text stating that on conflict with returning is not supported. I don't see the need for anything else. Do you?

@Amruta-Ranade, the language around returning nothing in the upsert and insert docs, and probably in other sql statement docs, needs to be revisited once your Parallel Statement Execution PR is further along.

[couchand]

@jseldess LGTM

I'll also draw your attention to cockroachdb/cockroach#17435, which I don't think needs anything in particular, since we hadn't documented the missing functionality anyway. But it might be nice, for those unfamiliar with the details of PGSQL and uninclined to dig into the SQL syntax page, to point out that the DO UPDATE can have a WHERE filter after the SET clause.

[jseldess]

TFTR, @couchand. I also added an example showing DO UPDATE SET with a WHERE filter and called it out as new in 1.1.

[cockroach-teamcity]

http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/0136fb6d318a029ab58daea0a83be5e87dd76856/

[jseldess]

TC only failing on what should be valid links to old cockroach prs. Overriding TC and merging in the hope that the GitHub hiccup will pass.