SGML improvements.

doc/src/sgml/admin.sgml

doc/src/sgml/charset.sgml

doc/src/sgml/client-auth.sgml

doc/src/sgml/datatype.sgml

doc/src/sgml/ddl.sgml

doc/src/sgml/diskusage.sgml

doc/src/sgml/ecpg.sgml

doc/src/sgml/features.sgml

doc/src/sgml/info.sgml

doc/src/sgml/installation.sgml

doc/src/sgml/jdbc.sgml

doc/src/sgml/libpgtcl.sgml

doc/src/sgml/libpq.sgml

doc/src/sgml/manage-ag.sgml

doc/src/sgml/mvcc.sgml

doc/src/sgml/programmer.sgml

doc/src/sgml/ref/create_cast.sgml

doc/src/sgml/ref/create_database.sgml

doc/src/sgml/ref/ecpg-ref.sgml

doc/src/sgml/ref/explain.sgml