Code review for pg_locks feature. Make shmemoffset of PROCLOCK structs

Code review for pg_locks feature.  Make shmemoffset of PROCLOCK structs
available (else there's no way to interpret the list links).  Change
pg_locks view to show transaction ID locks separately from ordinary
relation locks.  Avoid showing N duplicate rows when the same lock is
held multiple times (seems unlikely that users care about exact hold
count).  Improve documentation.