Document how to handle separators

[xxchan]

Just checked how sqllogictest variants handle separators in the results:

• sqlite: each row is one value instead of one row, so spaces are compared, but result cannot contain newline
• duckdb: tab separated, so spaces are compared, but result cannot contain newline and tab
• cockroach: compare "normalized rows", so whitespace characters are squashed. e.g., the following test will pass

query ITI
SELECT 1, E'a \t\n b', 2
----
1 a      b 2

[xxchan]

IIUC currently our behavior is the same as cockroach's. Not exactly, we use String instead of Vec<String> when parsing results. 🤪

I think cockroach's approach makes sense. slt seems not a good place to test things like whitespace characters. But we need to document it in case some users get surprised.

[xxchan]

BTW, we can add another control directive to tweak the behavior...