Fix comment about length of text, bytea, etc.
Fix comment about length of text, bytea, etc.

When commit 3e23b68dac006e8deb0afa327e855258df8de064 introduced
single-byte varlena headers, it rendered this comment incomplete.

50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb