[DOCS] Cross-references between DOMAIN and POLICY

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.6/static/sql-createpolicy.html
Description:

Domains and row level policies are similar in the definition of a constraint
with a CHECK expression.

There should be a mention of domains on the page `CREATE POLICY` and an
explanation of the differences between domains and policies in the
discussion of `Row Security Policies`.

There should be a mention of policies on the page `CREATE DOMAIN`.