docs: syntax.sgml patch

I've made an attempt to better describe the FROM clause and table expressions
in general for the SQL Syntax: Expressions page in the User's Guide.

My patch replaces the "FROM clause" section that was in there with a section
called "Table Expressions". It includes a description of the FROM clause and
also the WHERE, GROUP BY, HAVING, ORDER BY, and LIMIT clauses. The JOIN
syntax is covered. Subqueries are also explained some.

There could be some mistakes or statements that imply a limitation that is
not true (maybe). I encourage people to review it and correct any mistakes
or clean up the language etc.

If it's not right for being applied, let me know. I'm hoping this material
will fit in and be developed further with more examples later.

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