[DOC] pg_database_size/pg_tablespace_size error on a missing OID

Hi hackers,

The introduction to the database object size functions in
func-admin.sgml says that passing an OID that does not represent an
existing object returns NULL. That is true for the relation size
functions, but pg_database_size() and pg_tablespace_size() raise an
error for a nonexistent OID instead. The attached patch documents the
exception.

Thanks,
Shihao