"Features that differ from PostgreSQL" should mention NULL sort order

[This page lists most postgresql compatibility issues](https://www.cockroachlabs.com/docs/stable/postgresql-compatibility#features-that-differ-from-postgresql), but it doesn't mention that [NULLs are sorted differently by ORDER BY](https://www.cockroachlabs.com/docs/stable/null-handling#nulls-and-sorting). I think it should be mentioned

Also maybe it should explain how to use the ` null_ordered_last` session variable.

Jira issue: DOC-15693