The description of the lock type for speculative insertions was
incorrect, being copy-pasted from another one.

As discussed, also move the description for all the fields of lock tag
types from the structure listing lock tag types to the set of macros
setting each LOCKTAG.

Author: John Naylor
Discussion: https://postgr.es/m/CACPNZCtA0-ybaC4fFfaDq_8p_TUOLvGxZH9Dm-=TMHZJarBa7Q@mail.gmail.com