remove usage of "operator" to avoid c++ incompatibility
initial doc draft

what else needs doing here:

 - wordsmithing
 - check the sgml (currently untested)
 - add explanatory text and cross-references from, at least,
   "set returning functions" and "array functions" pointing to
   the table function query syntax
 - consider whether the existing ORDINALITY explanation should be
   moved to queries-tablefunctions (at the very least there needs
   to be cross-referencing)
 - any indexing markup needed?
 - maybe revise and improve the examples? the existing ones are not
   very good
 - anything else I didn't think of

more speculative:

 - maybe queries-from ought to be broken up into separate items?