Entity-ify a passel of & < > characters. Per gripe from Devrim.

doc/src/sgml/btree-gist.sgml

doc/src/sgml/cube.sgml

doc/src/sgml/datatype.sgml

doc/src/sgml/ddl.sgml

doc/src/sgml/dml.sgml

doc/src/sgml/earthdistance.sgml

doc/src/sgml/ecpg.sgml

doc/src/sgml/func.sgml

doc/src/sgml/fuzzystrmatch.sgml

doc/src/sgml/intarray.sgml

doc/src/sgml/isn.sgml

doc/src/sgml/lo.sgml

doc/src/sgml/ltree.sgml

doc/src/sgml/plpgsql.sgml

doc/src/sgml/pltcl.sgml

doc/src/sgml/ref/create_opclass.sgml

doc/src/sgml/release.sgml

doc/src/sgml/seg.sgml

doc/src/sgml/tablefunc.sgml

doc/src/sgml/trigger.sgml