Update hstore description.

Include a comparison to JSON.
Update the docs for the hstore format.

And document the GUCs that control the output format.
Add return value to operators table.
Docment new operators.

Excluding comparison operators, as they were never documented.
Document new functions.
Document replace() and concat_path().

Also note that @> and <@ go deep.
Fix SGML formatting issues.

Thanks Oleg!
Minor formatting changes.
Update compatibility note.
Pasto.
Add missing </para> tag
Document new hstore() constructors.

Better for composing scalar hstore values, nested hstores, and array hstores.
HSTORE arrays now always use brackets.
Root hash now always undecorated.
Document hstore_print().
Expand the populate_reocrd() docs.

Demonstrate nested value conversions.
Merge branch 'hstore-docs' into hstore