Lock tag types not in Index of library names

[geoffromer]

The following library names, all specified in [thread.lock]/p2, do not appear in the "Index of library names", as of n3376:

defer_lock_t
defer_lock
try_to_lock_t
try_to_lock
adopt_lock_t
adopt_lock

[jwakely]

The commit above (in my fork) would fix this, Stefanus is that the right place to put the index commands?

[sdutoit]

Commit looks good to me.